Method and apparatus for providing downlink control information in a MIMO wireless communication system

ABSTRACT

The present invention relates to a method in which a user equipment receives a downlink signal from a base station in a wireless communication system that supports downlink mimo transmission according to one embodiment of the present invention comprises: receiving downlink control information that includes information indicative of the number of layers (n, 1≤n≤8) where one or two enabled code words of the downlink mimo transmission are mapped; on the basis of the downlink control information, receiving downlink data transmitted over the respective n layers and a ue-specific reference signal for each of the n layers; and decoding the downlink data on the basis of the ue-specific reference signals, wherein the information indicative of the number of layers can further include information on a code for identifying the ue-specific reference signals.

This application is a Continuation of U.S. application Ser. No.14/797,560, filed Jul. 13, 2015, which is a Continuation of U.S.application Ser. No. 13/522,270 filed Jul. 13, 2012, which is a 35 USC §371 National Stage entry of International Application No.PCT/KR2011/000480, filed on Jan. 24, 2011, and claims the benefit ofU.S. Provisional Application No. 61/297,430, filed Jan. 22, 2010 andKorean Patent Application No. 10-2011-0006755 filed Jan. 24, 2011, eachof which are hereby incorporated by reference for all purposes as iffully set forth herein in their entireties.

TECHNICAL FIELD

The present invention relates to a wireless communication system, andmore particularly, to a method and apparatus for providing downlinkcontrol information in a multiple-input multiple-output (MIMO) wirelesscommunication system.

BACKGROUND ART

In a mobile communication system, a user equipment (UE) may receiveinformation from a base station in downlink and transmit information tothe base station in uplink. Information transmitted or received by a UEincludes data and a variety of control information and various physicalchannels are used according to the type and usage of the informationtransmitted or received by the UE.

A downlink channel may include a downlink control channel, a downlinkdata channel, etc. The downlink control channel may include a controlsignal defining resource allocation and transmission format of a signaltransmitted through the downlink data channel. Control informationtransmitted through the downlink control channel may be referred to asdownlink control information (DCI). The downlink control channel mayinclude a variety of DCI formats and the DCI format may include downlinkresource allocation information, uplink resource allocation information,etc.

Meanwhile, a multiple-input multiple-output (MIMO) scheme is applicablein uplink or downlink. The MIMO scheme refer to a technology ofutilizing two or more transmit/receive antennas in a transmitter and/ora receiver and spatially and simultaneously transmitting several datastreams so as to increase system capacity. As the MIMO scheme usingseveral transmission antennas, transmit diversity, spatial multiplexing,beamforming, etc. may be used.

If a MIMO scheme is applied to downlink transmission, in order toaccurately perform downlink transmission by a downlink receiver (e.g., aUE), there is a need for DCI for downlink MIMO transmission.

DISCLOSURE Technical Problem

In the conventional 3GPP LTE system, in downlink transmission, a maximumof two codewords may be supported, transmission through a maximum offour layers (that is, transmission of a maximum of rank 4) may besupported, and only single-layer beamforming transmission may besupported as a beamforming scheme. Introduction of systems for providingbetter performance than the conventional 3GPP LTE system have beendiscussed. These systems may support new MIMO schemes such as dual-layerbeamforming, transmission of a maximum of rank 8 and UE-specificreference signal (RS) based multi-user MIMO.

In case of downlink transmission using a new MIMO scheme different fromthe conventional MIMO scheme, a downlink receiver may not accuratelyreceive a downlink signal due to control information according to apreviously defined DCI format. Accordingly, there is a need fordesigning a DCI format for providing control information necessary toaccurately transmit and receive a downlink signal in a new downlink MIMOtransmission method. Accordingly, an object of the present invention isto provide a method and apparatus capable of providing downlink controlinformation necessary for new downlink MIMO transmission.

The technical problems solved by the present invention are not limitedto the above technical problems and other technical problems which arenot described herein will become apparent to those skilled in the artfrom the following description.

Technical Solution

The object of the present invention can be achieved by providing amethod of receiving, at a user equipment (UE), a downlink signal from abase station in a wireless communication system supporting downlinkmultiple-input multiple-output (MIMO) transmission, the method includingreceiving downlink control information including information indicatingthe number N (1≤N≤8) of layers, to which one or two enabled codewords ofthe downlink MIMO transmission are mapped, receiving downlink datatransmitted on the N layers and UE-specific reference signals of the Nlayers based on the downlink control information, and demodulating thedownlink data based on the UE-specific reference signals, wherein theinformation indicating the number of layers further includes informationabout codes for identifying the UE-specific reference signals.

According to another aspect of the present invention, there is provideda method of transmitting, at a base station, a downlink signal to a userequipment (UE) in a wireless communication system supporting downlinkmultiple-input multiple-output (MIMO) transmission, the method includingtransmitting downlink control information including informationindicating the number N (1≤N≤8) of layers, to which one or two enabledcodewords of the downlink MIMO transmission are mapped, and transmittingdownlink data transmitted on the N layers and UE-specific referencesignals of the N layers based on the downlink control information,wherein the downlink data is demodulated by the UE based on theUE-specific reference signals, wherein the information indicating thenumber of layers further includes information about codes foridentifying the UE-specific reference signals.

According to another aspect of the present invention, there is provideda user equipment (UE) for receiving a downlink signal from a basestation in a wireless communication system supporting downlinkmultiple-input multiple-output (MIMO) transmission, the UE including areception module configured to receive downlink control information anddownlink data from the base station, a transmission module configured totransmit uplink control information and uplink data to the base station,and a processor configured to control the UE including the receptionmodule and the transmission module, wherein the processor receives thedownlink control information including information indicating the numberN (1≤N≤8) of layers, to which one or two enabled codewords of thedownlink MIMO transmission are mapped, through the reception module,receives downlink data transmitted on the N layers and UE-specificreference signals of the N layers based on the downlink controlinformation through the reception module, and demodulates the downlinkdata based on the UE-specific reference signals, and wherein theinformation indicating the number of layers further includes informationabout codes for identifying the UE-specific reference signals.

According to another aspect of the present invention, there is provideda base station for transmitting a downlink signal to a user equipment(UE) in a wireless communication system supporting downlinkmultiple-input multiple-output (MIMO) transmission, the base stationincluding a reception module configured to receive uplink controlinformation and uplink data from the UE, a transmission moduleconfigured to transmit downlink control information and downlink data tothe UE, and a processor configured to control the base station includingthe reception module and the transmission module, wherein the processortransmits the downlink control information including informationindicating the number N (1≤N≤8) of layers, to which one or two enabledcodewords of the downlink MIMO transmission are mapped, through thetransmission module, and transmits downlink data transmitted on the Nlayers and UE-specific reference signals of the N layers based on thedownlink control information through the transmission module, whereinthe downlink data is demodulated by the UE based on the UE-specificreference signals, and wherein the information indicating the number oflayers further includes information about codes for identifying theUE-specific reference signals.

In the embodiments of the present invention, the information about thecodes for identifying the UE-specific reference signals may be includedin the information indicating the number of layers only when onecodeword is mapped to one layer and when two codewords are mapped to twolayers.

In the embodiments of the present invention, the information about thecodes for identifying the UE-specific reference signals may distinguishthe UE-specific reference signals transmitted at the same resourceelement position.

In the embodiments of the present invention, four different UE-specificreference signals transmitted at the same resource element position maybe included in one reference signal group, and one reference signalgroup may be divided into two subgroups by the information about thecodes for identifying the UE-specific reference signals, and onesubgroup may include two UE-specific reference signals divided byorthogonal codes.

In the embodiments of the present invention, the downlink controlinformation may further include information indicating antenna ports ofthe downlink MIMO transmission.

In the embodiments of the present invention, the information indicatingthe number of layers may have 3 bits.

The above general description and the following detailed description ofthe present invention are exemplary and are intended to additionallydescribe the claims.

Advantageous Effects

According to the present invention, it is possible to provide a methodand apparatus for providing control information of downlink transmissionin a wireless communication system to which transmission of a maximum ofrank 8, dual-layer beamforming, UE-specific reference signal basedmulti-user MIMO, etc. is applied.

The effects of the present invention are not limited to theabove-described effects and other effects which are not described hereinwill become apparent to those skilled in the art from the followingdescription.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiment(s) of the invention andtogether with the description serve to explain the principle of theinvention.

FIG. 1 is a diagram showing the structure of a radio frame used in a3^(rd) Generation Partnership Project Long Term Evolution (3GPP LTE)system.

FIG. 2 is a diagram showing a resource grid of a downlink slot.

FIG. 3 is a diagram showing the structure of a downlink subframe.

FIG. 4 is a diagram showing the structure of an uplink subframe.

FIG. 5 is a block diagram showing a MIMO transmission structure.

FIG. 6 is a diagram showing a mapping relationship between a layer and aphysical antenna in a MIMO transmission structure.

FIG. 7 is a diagram illustrating a reference signal pattern in a 3GPPLTE system.

FIGS. 8 and 9 are diagrams illustrating a demodulation reference signal(DMRS) pattern.

FIG. 10 is a diagram illustrating a method of transmitting and receivinga downlink signal according to an embodiment of the present invention.

FIG. 11 is a diagram showing the configuration of a base station and aUE according to an exemplary embodiment of the present invention.

BEST MODE

The following embodiments are proposed by combining constituentcomponents and characteristics of the present invention according to apredetermined format. The individual constituent components orcharacteristics should be considered optional on the condition thatthere is no additional remark. If required, the individual constituentcomponents or characteristics may not be combined with other componentsor characteristics. Also, some constituent components and/orcharacteristics may be combined to implement the embodiments of thepresent invention. The order of operations to be disclosed in theembodiments of the present invention may be changed. Some components orcharacteristics of any embodiment may also be included in otherembodiments, or may be replaced with those of the other embodiments asnecessary.

The embodiments of the present invention are disclosed on the basis of adata communication relationship between a base station and a userequipment (UE). In this case, the base station is used as a terminalnode of a network via which the base station can directly communicatewith the terminal. Specific operations to be conducted by the basestation in the present invention may also be conducted by an upper nodeof the base station as necessary.

In other words, it will be obvious to those skilled in the art thatvarious operations for enabling the base station to communicate with theterminal in a network composed of several network nodes including thebase station will be conducted by the base station or other networknodes other than the base station. The term “Base Station (BS)” may bereplaced with the terms fixed station, Node-B, eNode-B (eNB), or accesspoint as necessary. In the present specification, the term “basestation” may include a cell or a sector. The term “relay” may bereplaced with the terms relay node (RN) or relay station (RS). The term“terminal” may also be replaced with the terms User Equipment (UE),Mobile Station (MS), Mobile Subscriber Station (MSS) or SubscriberStation (SS) as necessary.

In the present invention, a downlink transmitter may be a base stationor a relay (if the relay transmits access downlink to a UE) and adownlink receiver may be a UE or a relay (if the relay receives backhauldownlink from a base station). In the following description, although abase station will be representatively described as a downlinktransmitter and a UE will be representatively described as a downlinkreceiver, the present invention is not limited thereto and is applicableto any downlink transmitter and receiver.

It should be noted that specific terms disclosed in the presentinvention are proposed for the convenience of description and betterunderstanding of the present invention, and the use of these specificterms may be changed to another format within the technical scope orspirit of the present invention.

In some instances, well-known structures and devices are omitted inorder to avoid obscuring the concepts of the present invention and theimportant functions of the structures and devices are shown in blockdiagram form. The same reference numbers will be used throughout thedrawings to refer to the same or like parts.

Exemplary embodiments of the present invention are supported by standarddocuments disclosed for at least one of wireless access systemsincluding an Institute of Electrical and Electronics Engineers (IEEE)802 system, a 3^(rd) Generation Project Partnership (3GPP) system, a3GPP Long Term Evolution (LTE) system, and a 3GPP2 system. Inparticular, the steps or parts, which are not described to clearlyreveal the technical idea of the present invention, in the embodimentsof the present invention may be supported by the above documents. Allterminology used herein may be supported by at least one of theabove-mentioned documents.

The following technologies can be applied to a variety of wirelessaccess technologies, for example, CDMA (Code Division Multiple Access),FDMA (Frequency Division Multiple Access), TDMA (Time Division MultipleAccess), OFDMA (Orthogonal Frequency Division Multiple Access), SC-FDMA(Single Carrier Frequency Division Multiple Access), and the like. CDMAmay be embodied as wireless (or radio) technology such as UTRA(Universal Terrestrial Radio Access) or CDMA2000. TDMA may be embodiedwith wireless (or radio) technology such as GSM (Global System forMobile communications)/GPRS (General Packet Radio Service)/EDGE(Enhanced Data Rates for GSM Evolution). OFDMA may be embodied withwireless (or radio) technology such as Institute of Electrical andElectronics Engineers (IEEE) 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE802-20, and E-UTRA (Evolved UTRA). UTRA is a part of the UMTS (UniversalMobile Telecommunications System). 3GPP (3rd Generation PartnershipProject) LTE (long term evolution) is a part of the E-UMTS (EvolvedUMTS), which uses E-UTRA. 3GPP LTE employs the OFDMA in downlink andemploys the SC-FDMA in uplink. LTE-Advanced (LTE-A) is an evolvedversion of 3GPP LTE. WiMAX can be explained by an IEEE 802.16e(WirelessMAN-OFDMA Reference System) and an advanced IEEE 802.16m(WirelessMAN-OFDMA Advanced System). For clarity, the followingdescription focuses on 3GPP LTE and LTE-A. However, the technical spiritof the present invention is not limited thereto.

FIG. 1 is a diagram showing the structure of a radio frame used in a3^(rd) Generation Partnership Project Long Term Evolution (3GPP LTE)system. One downlink frame includes 10 subframes, and one subframeincludes two slots in a time domain. A time required for transmittingone subframe is referred to as a Transmission Time Interval (TTI). Forexample, one subframe may have a length of 1 ms and one slot may have alength of 0.5 ms. One slot may include a plurality of OFDM symbols in atime domain. In the 3GPP LTE system, since an OFDMA scheme is used indownlink, the OFDM symbol indicates one symbol period. One symbol may becalled an SC-FDMA symbol or symbol period in uplink. A resource block(RB) is a resource allocation unit and includes a plurality ofcontiguous subcarriers in one slot. The structure of the radio frame isonly exemplary. Accordingly, the number of subframes included in theradio frame, the number of slots included in the subframe or the numberof OFDM symbols included in the slot may be changed in various manners.

FIG. 2 is a diagram showing a resource grid of a downlink slot. Onedownlink slot includes seven OFDM symbols in a time domain and one RBincludes 12 subcarriers in a frequency domain, to which the presentinvention is not limited. For example, one slot includes seven OFDMsymbols in a normal cyclic prefix (CP) and one slot includes six OFDMsymbols in an extended CP. Each element of the resource grid is referredto as a Resource Element (RE). One RB includes 12×7 REs. The numberN^(DL) of RBs included in the downlink slot is determined based ondownlink transmission bandwidth. The structure of an uplink slot may beequal to that of the downlink slot.

FIG. 3 is a diagram showing the structure of a downlink subframe. Amaximum of three OFDM symbols of a front portion of a first slot withinone subframe corresponds to a control region to which control channelsare allocated. The remaining OFDM symbols correspond to a data region towhich Physical Downlink Shared Channels (PDSCHs) are allocated. Examplesof the downlink control channels used in the 3GPP LTE system include,for example, a Physical Control Format Indicator Channel (PCFICH), aPhysical Downlink Control Channel (PDCCH), a Physical Hybrid automaticrepeat request Indicator Channel (PHICH), etc. The PCFICH is transmittedat a first OFDM symbol of a subframe, and includes information about thenumber of OFDM symbols used to transmit the control channel in thesubframe. The PHICH includes a HARQ ACK/NACK signal as a response touplink transmission. The control information transmitted through thePDCCH is referred to as Downlink Control Information (DCI).

The DCI includes uplink or downlink scheduling information or an uplinktransmit power control command for a certain UE group. The PDCCH mayinclude resource allocation and transmission format of a Downlink SharedChannel (DL-SCH), resource allocation information of an Uplink SharedChannel (UL-SCH), paging information of a Paging Channel (PCH), systeminformation on the DL-SCH, resource allocation of a higher layer controlmessage such as a Random Access Response (RAR) transmitted on the PDSCH,a set of transmit power control commands for individual UEs in a certainUE group, transmit power control information, activation of Voice overIP (VoIP), etc. The PDCCH is transmitted on an aggregation of one orseveral consecutive control channel elements (CCEs). The CCE is alogical allocation unit used to provide the PDCCHs at a coding ratebased on the state of a radio channel. The CCE corresponds to aplurality of resource element groups. The format of the PDCCH and thenumber of available bits are determined based on a correlation betweenthe number of CCEs and the coding rate provided by the CCEs. The basestation determines a PDCCH format according to a DCI to be transmittedto the terminal, and attaches a Cyclic Redundancy Check (CRC) to controlinformation. The CRC is masked with a Radio Network Temporary Identifier(RNTI) according to an owner or usage of the PDCCH. If the PDCCH is fora specific terminal, a cell-RNTI (C-RNTI) of the terminal may be maskedto the CRC. Alternatively, if the PDCCH is for a paging message, apaging indicator identifier (P-RNTI) may be masked to the CRC. If thePDCCH is for system information (more specifically, a system informationblock (SIB)), a system information identifier and a system informationRNTI (SI-RNTI) may be masked to the CRC. To indicate a random accessresponse that is a response for transmission of a random access preambleof the terminal, a random access-RNTI (RA-RNTI) may be masked to theCRC.

A plurality of PDCCHs may be transmitted within the control region. A UEmay monitor the plurality of PDCCHs. Monitoring indicates that a UEattempts to decode PDCCHs according to a DCI format. In a control regionallocated within a subframe, a base station does not provide the UE withinformation about where the PDCCHs are located. The UE monitors a set ofPDCCH candidates within the subframe and finds PDCCHs thereof. This iscalled blind decoding. For example, if the UE does not detect CRC errorby demasking a C-RNTI thereof in the PDCCHs, the UE detects a PDCCHhaving DCI thereof. The UE may be set to receive PDSCH data transmissionsignaled via a PDCCH according to various transmission modes and suchsettings may be semi-statically designated via higher layer signaling.

FIG. 4 is a diagram showing the structure of an uplink subframe. Theuplink subframe may be divided into a control region and a data regionin a frequency domain. A Physical Uplink Control Channel (PUCCH)including uplink control information is allocated to the control region.A Physical uplink Shared Channel (PUSCH) including user data isallocated to the data region. In order to maintain single carriercharacteristics, one UE does not simultaneously transmit the PUCCH andthe PUSCH. The PUCCH for one UE is allocated to a RB pair in a subframe.RBs belonging to the RB pair occupy different subcarriers with respectto two slots. Thus, the RB pair allocated to the PUCCH is“frequency-hopped” at a slot edge.

The MIMO system uses a technology of collecting data pieces received viaseveral antennas without depending on a single antenna path in order toreceive one message. According to the MIMO technology, since datatransmission rate is improved in a specific range or a system range canbe increased with respect to a specific data transmission rate, the MIMOtechnology may be widely used for a mobile communication UE and a relay.MIMO may also be multi-antenna technology.

A MIMO channel matrix according to the number of receive antennas andthe number of transmit antennas may be divided into a plurality ofindependent channels. Each independent channel is referred to as a layeror stream. Rank may mean the number of layers or streams. In thefollowing description, in MIMO transmission, “rank” indicates the numberof paths which can independently transmit a signal and “the number oflayers” indicates the number of signal streams transmitted through eachpath. In general, since a transmitter transmits layers corresponding innumber to the number of ranks used for signal transmission, rank has thesame meaning as number of layers unless otherwise stated.

As a MIMO scheme using several transmit antennas, transmit diversity,spatial multiplexing, beamforming, etc. may be used.

The transmit diversity scheme has an advantage that high-reliabilitydata transmission is implemented without receiving feedback informationassociated with a channel from a receiver by transmitting the same datainformation through several transmit antennas.

Beamforming is used to increase a signal to interference plus noiseratio (SINR) of a receiver by multiplying several transmit antennas bysuitable weights. In general, since an uplink/downlink channel isindependent in a frequency division duplexing system and thushigh-reliability channel information is necessary in order to obtainsuitable beamforming gain, separate feedback may be received from areceiver in order to apply beamforming.

A spatial multiplexing scheme may be divided into a single-user spatialmultiplexing scheme and a multi-user spatial multiplexing scheme. Thesingle-user spatial multiplexing scheme is called a spatial multiplexing(SM) scheme or an SU-MIMO scheme, which allocates a plurality of antennaresources of a base station to one user (UE). MIMO channel capacity isincreased in proportion to the number of antennas. The multi-userspatial multiplexing scheme is called a spatial division multiple access(SDMA) scheme or an MU-MIMO scheme, which distributes a plurality ofantenna resources or wireless spatial resources of a base station to aplurality of users (UEs).

In the case of using the MIMO scheme, there are a single codeword (SCW)scheme for simultaneously transmitting N data streams using one channelencoding block and a multiple codeword (MCS) scheme for transmitting Ndata streams using M (M being always less than or equal to N) channelencoding blocks. At this time, each channel encoding block generatesindependent codewords and each codeword may be designed to enableindependent error detection.

The conventional MIMO system is designed based on the MCW structure. Inthe MCW structure, a maximum of two codewords is allowed to besimultaneously transmitted. For MIMO transmission, modulation and codingscheme (MCS) information used by a transmitter, a new data indicator(NDI) indicating whether transmitted data is new data or retransmitteddata, and redundancy version (RV) information indicating which subpacketis retransmitted in case of retransmission, etc. are necessary.

FIG. 5 is a block diagram showing a MIMO transmission structure. In asystem supporting a MIMO scheme, a transmitter may transmit one or morecodewords. The codewords are mapped to transport blocks from a higherlayer, which will be described below. FIG. 5 shows a system supporting amaximum of two codewords. One or more codewords may be processed ascomplex symbols through a scrambling module and a modulation mapper.Thereafter, the complex symbols are mapped to a plurality of layers by alayer mapper and each layer is multiplied by a predetermined precodingmatrix selected according to a channel state by a precoding module to beallocated to each transmit antenna. The signals transmitted through therespective antennas are mapped to time-frequency resource elements to beused for transmission by resource element mappers and are transmittedthrough OFDM signal generators and antennas.

A transport block-to-codeword mapping relationship will now bedescribed. In FIG. 5, two transport blocks (TBs) are mapped to twocodewords according to a transport block-to-codeword mapping rule. Iftwo TBs are enabled, TB-to-codeword mapping may be swapped according toa TB-to-CW swap flag. The TB-to-CW mapping rule may be configured asshown in Tables 1 and 2.

TABLE 1 TB to CW swap flag value CW 0 (enabled) CW 1 (enabled) 0 TB 1 TB2 1 TB 2 TB 1

TABLE 2 TB 1 TB 2 CW 0 (enabled) CW 1 (disabled) enabled disabled TB 1 —disabled enabled TB 2 —

Table 1 shows an example of a TB-to-CW mapping rule when two TBs areenabled and Table 2 shows an example of a TB-to-CW mapping rule when oneTB is enabled and the other is disabled.

Disabling of the TB in Table 2 includes the case in which the size ofthe TB is 0. If the size of the TB is 0, the TB is not mapped to the CW.

If a signal is transmitted using a single antenna, one codeword ismapped to one layer and is transmitted. However, if a signal istransmitted using multiple antennas, a codeword-to-layer mapping rule isdefined as shown in Tables 3 and 4 according to a transmission scheme.

TABLE 3 Number of Number of Codeword-to-layer mapping layers code wordsi = 0, 1, . . . , M_(symb) ^(layer) − 1 1 1 x⁽⁰⁾(i) = d⁽⁰⁾(i) M_(symb)^(layer) = M_(symb) ⁽⁰⁾ 2 2 x⁽⁰⁾(i) = d⁽⁰⁾(i) M_(symb) ^(layer) =M_(symb) ⁽⁰⁾ = x⁽¹⁾(i) = d⁽¹⁾(i) M_(symb) ⁽¹⁾ 2 1 x⁽⁰⁾(i) = d⁽⁰⁾(2i)M_(symb) ^(layer) = M_(symb) ⁽⁰⁾/2 x⁽¹⁾(i) = d⁽⁰⁾(2i + 1) 3 2 x⁽⁰⁾(i) =d⁽⁰⁾(i) M_(symb) ^(layer) = M_(symb) ⁽⁰⁾ = x⁽¹⁾(i) = d⁽¹⁾(2i) M_(symb)⁽¹⁾/2 x⁽²⁾(i) = d⁽¹⁾(2i + 1) 4 2 x⁽⁰⁾(i) = d⁽⁰⁾(2i) M_(symb) ^(layer) =M_(symb) ⁽⁰⁾/2 = x⁽¹⁾(i) = d⁽⁰⁾(2i + 1) M_(symb) ⁽¹⁾/2 x⁽²⁾(i) =d⁽¹⁾(2i) x⁽³⁾(i) = d⁽¹⁾(2i + 1)

TABLE 4 Number of Number code Codeword-to-layer mapping of layers wordsi = 0, 1, . . . , M_(symb) ^(layer) − 1 2 1 x⁽⁰⁾ (i) = d⁽⁰⁾ (2i) x⁽¹⁾(i) = d⁽⁰⁾ (2i + 1) M_(symb) ^(layer) = M_(symb) ⁽⁰⁾/2 4 1 x⁽⁰⁾ (i) =d⁽⁰⁾ (4i) x⁽¹⁾ (i) = d⁽⁰⁾ (4i + 1) x⁽²⁾ (i) = d⁽⁰⁾ (4i + 2) x⁽³⁾ (i) =d⁽⁰⁾ (4i + 3) $M_{symb}^{layer} = \{ {{\begin{matrix}{M_{symb}^{(0)}/4} & {{{if}\mspace{14mu} M_{symb}^{(0)}\mspace{14mu}{mod}\mspace{14mu} 4} = 0} \\{( {M_{symb}^{(0)} + 2} )/4} & {{{if}{\mspace{11mu}\;}M_{symb}^{(0)}\mspace{14mu}{mod}\mspace{14mu} 4} \neq 0}\end{matrix}{If}\mspace{14mu} M_{symb}^{(0)}\mspace{14mu}{mod}\mspace{14mu} 4} \neq {0\mspace{14mu}{two}\mspace{14mu}{null}\mspace{14mu}{symbols}\mspace{14mu}{shall}{\mspace{11mu}\;}{be}{appended}{\mspace{11mu}\;}{to}\mspace{14mu}{d^{(0)}( {M_{symb}^{(0)} - 1} )}}} $

Table 3 shows an example in which a signal is transmitted using aspatial multiplexing method and Table 4 shows an example in which asignal is transmitted using a transmit diversity method. In Tables 3 and4, x^((a))(i) indicates an i-th symbol of a layer having an index a andd^((a))(i) indicates an i-th symbol of a codeword having an index a. Amapping relationship between the number of codewords and the number oflayers used for transmission may be confirmed through the item “numberof layers” and the item “number of codewords” of Tables 3 and 4, and theitem “codeword-to-layer mapping” indicates how symbols of codewords arerespectively mapped to layers.

As can be seen from Tables 3 and 4, although one codeword may be mappedto one layer and transmitted in symbol units, one codeword may bedistributed and mapped to a maximum of four layers as in the second caseof Table 4.

A mapping relationship between a layer and a physical antennal will bedescribed with reference to FIG. 6. The following description isexemplary and the mapping relationship between the layer and thephysical antenna may be arbitrary. In the following description, it isassumed that a system supporting a MIMO scheme has, for example, fourphysical transmit antennas. If rank is 1, one codeword CW1 may be mappedto one layer and data made by one layer may be encoded using a precodingmethod so as to be transmitted through four transmit antennas. If rankis 2, two codewords CW1 and CW2 are mapped to two layers and are mappedto four transmit antennas using a precoder. If rank is 3, one codewordCW1 among two codewords is mapped to one layer and the other codewordCW2 is mapped to two layers by a serial-parallel (S/P) converter. Thatis, a total of two codewords is mapped to three layers and is mapped tofour transmit antennas using a precoder. If rank is 4, each of twocodewords CW1 and CW2 is mapped to two layers by an S/P converter and atotal of four layers is mapped to four transmit antennas using aprecoder.

Although a base station having four transmit antennas may have a maximumof four layers and four independent codewords, FIG. 6 shows, forexample, a system in which the number of codewords is a maximum of 2. Asdescribed above, positions of information transmitted through twocodewords CW1 and CW2 may be changed.

Meanwhile, the precoder is generally expressed by Mt (the number oftransmit antennas)*v (a spatial multiplexing rate) and a precodingmatrix is adaptively used according to circumstance using a set ofmatrices predetermined by a transmitter/receiver. This set of precodingmatrices is referred to as a codebook.

In the conventional 3GPP LTE system, four or more logical antenna ports(e.g., antenna ports 0 to 5) may be used. The antenna ports are notphysically distinguished and a determination as to which physicalantenna indexes logical antenna indexes are mapped is made bymanufacturers. The antenna ports and the physical antennas may notcorrespond one-to-one to each other and one antenna port may correspondto one physical antenna or an antenna array which is a combination of aplurality of physical antennas.

In the 3GPP LTE system, as a downlink reference signal, three referencesignals including a cell-specific reference signal (which is notassociated with MBSFN transmission), an MBSFN reference signal andUE-specific reference signal associated with MBSFN transmission may beused.

The cell-specific reference signal uses a sequence generated using acell ID of each cell as an initial value and antenna ports 0 to 3 may beused to transmit the cell-specific reference signal. The MBSFN referencesignal is used to acquire downlink channel information for MBSFNtransmission and is transmitted via antenna port 4.

The UE-specific reference signal is supported for single antenna porttransmission of a PDSCH and may be transmitted via antenna port 5. Aterminal (UE) may receive information as to whether such a UE-specificreference signal from a higher layer (an MAC layer or a higher layer) isused for PDSCH demodulation. The UE-specific reference signal enablesbeamforming of data transmission to a specific UE. For example, a basestation may generate directional transmission to a specific UE using anarray (one antenna port) of adjacently located physical antennas. Thephases of the signals of different physical antennas are appropriatelyset and combined at the position of a UE. The UE recognizes suchdirectional transmission as transmission using one antenna. Since beamsformed by beamforming experience different channel responses between thebase station and the UE, there is a need for use of the UE-specificreference signal in order to enable the UE to accurately demodulatebeamformed data.

The above-described UE-specific reference signal corresponds to adedicated reference signal (DRS) or a precoded demodulation referencesignal (DMRS). In case of using a precoded reference signal, referencesignals corresponding in number to a spatial multiplexing rate aretransmitted.

The UE-specific reference signal may be used for single-layerbeamforming (beamforming of transmission of rank 1). As described above,since the UE-specific reference signal is precoded by the same precoderas a precoder applied to data on a PDSCH, a precoding matrix istransparent to the UE. That is, in case of transmission using theUE-specific reference signal, since an estimated channel includes aprecoding weight, single-layer beamforming may be implemented withoutprecoding information. Accordingly, a DCI format composed of controlsignal information which does not include precoding information may beused for single-layer beamforming. For example, for single-layerbeamforming, among the above-described DCI formats, DCI format 1 or DCIformat 1A defined for single antenna port transmission and transmitdiversity may be used.

Since only antenna port 5 is defined as an antenna port for transmittinga UE-specific reference signal in the conventional 3GPP LTE (Release-8)system, if rank is 2 or more, it is necessary to transmit data using thecell-specific reference signal (antenna ports 0 to 3). That is, each UEmay perform data demodulation using precoding weight informationacquired via a control channel and channel information acquired via acell-specific reference signal.

Recently, in 3GPP LTE Release-9, introduction of dual-layer beamforming(or dual stream beamforming) has been discussed. Dual-layer beamformingmeans a MIMO transmission scheme supporting transmission of a maximum ofrank 2 based on the UE-specific reference signal (DRS or DMRS) andcorresponds to extension of single-layer beamforming. According todual-layer beamforming, a maximum of two enabled TBs is respectivelymapped to two codewords and is transmitted via two layers and a DRS istransmitted per layer. According to dual-layer beamforming, even whenthe base station does not inform each UE of precoding information, theUE may receive MIMO transmission from the base station withoutmulti-user interference using channel information acquired via theUE-specific reference signal transmitted per layer.

A DRS for dual-layer beamforming may be designed such that layers areorthogonal using a time division multiplexing (TDM)/frequency divisionmultiplexing (FDM)/code division multiplexing (CDM) scheme. Iftransmission is performed using a single layer only, the UE is informedof a reference signal corresponding to a layer for performing singlelayer transmission among dedicated reference signals supporting twolayers so as to improve data demodulation performance. Accordingly,there is a need for a bit field indicating a reference signal used forsingle-layer beamforming in downlink control information.

In addition, in dual-layer beamforming, data may be transmitted andreceived via two layers or a single layer. The case of transmittingdifferent codewords via two layers corresponds to MCW SU-MIMO. In caseof transmission using a single layer, SU-MIMO or MU-MIMO may beperformed. The case of transmitting data to one user using a singlelayer corresponds to SU-MIMO. The case of allocating two layers todifferent users corresponds to MU-MIMO. In case of MU-MIMO, since eachUE may separate layers using channel information acquired via theUE-specific reference signal, the base station may provide informationindicating a layer corresponding to each UE such that the UE acquires achannel. As described above, in a dual-layer beamforming scheme, since amaximum of two layers is used, 1-bit information is necessary for thebase station to indicate one of two layers.

As a control signal for SU-MIMO dual-layer beamforming transmission, DCIformat 2A may be used. Meanwhile, a DCI format supporting dual-layerbeamforming based MU-MIMO should be determined. In the light of supportof SU-MIMO dual-layer beamforming and MU-MIMO dynamic switching, one DCIformat for SU-MIMO and MU-MIMO dual-layer beamforming transmission ispreferably defined so as to distinguish between SU-MIMO and MU-MIMOwithin the DCI format.

The operation of the 3GPP LTE Release-9 system may be defined to beincluded in the operation of the Release-10 system. That is, adual-layer beamforming transmission operation defined in 3GPP LTERelease-9 needs to be defined to be performed without any problems evenin the 3GPP LTE Release-10 system. Overhead of a DRS pattern (that is,the number of downlink resource elements in which a DRS is transmitted)of the 3GPP LTE Release-10 system is changed according to transmissionrank. In the 3GPP LTE Release-10 system, for example, a DRS pattern maybe designed such that DRS overhead is equally maintained from rank 1 torank 2, DRS overhead of rank 3 or more is increased as compared to DRSoverhead of rank 1 to rank 2, and DRS overhead is the same in rank 3 torank 8. Alternatively, a DRS pattern may be designed such that DRSoverhead is the same from rank 1 to rank 4, DRS overhead of rank 5 ormore is increased as compared to DRS overhead of rank 1 to rank 4 andDRS overhead is the same in rank 5 to rank 8. In the 3GPP LTE Release-9system, dual-layer beamforming is designed to have a maximum of rank 2and, in the MU-MIMO scheme, one layer is allocated to each UE. At thistime, even in the MU-MIMO operation, DRS overhead of rank 2 may bemaintained. Meanwhile, in the 3GPP LTE Release-10 system, sincetransmission of a maximum of rank 8 is supported, 8 or less layers maybe allocated to each UE when MU-MIMO transmission is applied.Accordingly, in case of transmission of a maximum of rank 8, the UEshould be informed of information about a total transmission rank usedfor transmission, a transmission rank of each UE and a layer allocatedto each UE in order to accurately perform MU-MIMO.

The DRS pattern of the 3GPP LTE Release-9 system may be composed of asubset of a DRS pattern of Release-10 and a transmission mode defined inRelease-9 may be defined as one of transmission modes defined inRelease-10. In this case, a DCI format only for dual-layer beamformingof Release-9 may be configured. Hereinafter, design of a DCI formatsimultaneously supporting SU-MIMO and MU-MIMO in a transmission mode ofRelease-9 will be described.

Bit Field Configuration Method of DCI Format

Hereinafter, an embodiment of the present invention for newlyinterpreting a field defined in the existing DCI format in order tosupport dual-layer beamforming will be described. In the existing 3GPPLTE standard (e.g., 3GPP LTE Release-8), DCI formats 0, 1, 1A, 1B, 1C,1D, 2, 2A, 3 and 3A are defined. Briefly, DCI format 0 is for uplinkresource allocation information, DCI formats 1 to 2 are for downlinkresource allocation information, and DCI formats 3 and 3A are for anuplink transmit power control (TPC) command for arbitrary UE groups.

DCI format 2A defined in the existing 3GPP LTE standard (Release-8) isshown in Table 5.

TABLE 5 DCI format 2A Resource Block Assignment N bits TPC command forPUCCH 2 bits Downlink Assignment Index 2 bits HARQ process number 3bits(FDD), 4 bits(TDD) Transport Block to codeword swap flag 1 bitTransport MCS 5 bits block 1 New data Indicator 1 bit Redundancy version2 bits Transport MCS 5 bits block 2 New data Indicator 1 bit Redundancyversion 2 bits Precoding information 0 bit (2Tx) 2 bits (4Tx)

DCI format 2A corresponds to a control information format for 2-codewordopen-loop spatial multiplexing transmission. Open-loop spatialmultiplexing transmission means that spatial multiplexing transmissionis implemented without feedback from a UE. Therefore, non-channeldependent precoding is used in the open loop scheme.

A transmission mode may be semi-statically changed through higher layersignaling. If a transmission mode is defined as dual-layer beamforming,a UE may interpret bit fields of DCI format 2A as having a meaningdifferent from control information for open-loop spatial multiplexingtransmission.

DCI format 2A supports a maximum of two codewords (transport blocks) andMCS, NDI and RV are defined with respect to each transport block. Asdescribed above, MCS is information about a modulation and coding schemeused by a transmitter, NDI is a new data indicator indicating whethertransmitted data is new data or retransmitted data, and RV is redundancyversion information indicating which subpacket is retransmitted in caseof retransmission.

2^(N) fields of MCS composed of N bits may be expressed by a modulationorder and a transport block size. Some fields are used to indicate anMCS used for data transmission and some fields express only a modulationorder, which may be used upon retransmission. In addition, some fieldsindicate a transport block size of “0”, which means that datatransmission is not performed. If an MCS field having a transport blocksize of “0” (zero) is indicated, this means that a transport block isdisabled. Alternatively, if an MCS field having a non-zero transportblock size is indicated, it means that a transport block is enabled.

A TB-to-CW swap flag indicates whether mapping relationships between twoTBs and two CWs may be swapped as described with reference to Table 1.

Precoding information defined in DCI format 2A provides informationabout transmission rank. Precoding information is set to 0 bits (thatis, the precoding information is not present) in case of transmissionusing 2 antenna ports and is set to 2 bits in case of transmission using4 antenna ports. The content of the precoding information fields of fourantenna ports may be defined as shown in Table 6.

TABLE 6 One codeword: Two codewords: Codeword 0 enabled, Codeword 0enabled, Codeword 1 disabled Codeword 1 enabled Bit field Bit fieldmapped mapped to index Message to index Message 0 4 layers: Transmit 0 2layers: precoder diversity cycling with large delay CDD 1 2 layers:precoder 1 3 layers: precoder cycling with large cycling with largedelay CDD delay CDD 2 reserved 2 4 layers: precoder cycling with largedelay CDD 3 reserved 3 reserved

In DCI format 2A, transmit diversity transmission is performed in caseof rank 1 (if one codeword is enabled) and spatial multiplexingtransmission having two codewords is performed in case of rank 2. Themethod of applying transmit diversity or spatial multiplexing accordingto rank may be determined as shown in Tables 7 and 8.

TABLE 7 2Tx Transmit diversity 4Tx Rank 1 Transmit diversity Rank 2Spatial Multiplexing

FIG. 8 2Tx Spatial Multiplexing 4Tx Rank 2 Spatial Multiplexing Rank 3Spatial Multiplexing Rank 4 Spatial Multiplexing

Table 7 shows the case in which any one TB is enabled and Table 8 showsthe case in which two TBs are both enabled. The TB-to-CW swap flag isnot necessary in case of Table 7, but the TB-to-codeword swap flag isnecessary in case of Table 8 in order to determine to which CWs two TBsare respectively mapped.

Rank may be explicitly or implicitly indicated. Explicit rank indicationmay be made using a method of defining a separate rank indicator.

A UE may acquire rank information using an implicit method withoutsetting a rank indicator. Enabling/disabling of TB may be indicated byMCS information and RV of the TB. In DCI format 2A, for example, if anMCS index value of a TB is set to 0 so as to indicate that a transportblock size is 0, transmission is not performed and thus the TB isdisabled. If the transport block size is not 0, it may be indicated thatthe TB is enabled. Alternatively, if the MCS index value is set to 0 andRV is set to 1, it may be indicated that the TB is disabled and,otherwise, it may be indicated that the TB is enabled. Accordingly, theUE implicitly confirms that transmission of rank 2 is performed if twoTBs are enabled and transmission of rank 1 is performed if one TB isenabled and the other TB is disabled.

According to definition of the existing DCI 2A format, if only one TB isenabled, the TB-to-CW swap flag is reserved and thus the TB-to-CW swapflag is interpreted as having a different meaning.

For example, in case of transmission of rank 1, the TB-to-CW swap flagof DCI format 2A may be used as information indicating an index for aTB, CW or layer used for single-layer beamforming.

Alternatively, the TB-to-CW swap flag may be used as an indicatorindicating whether DRS overhead is changed in use of the DRS defined in3GPP LTE Release-9 and Release-10. For example, in the case in which DRSoverhead of 3GPP LTE Release-10 is increased as compared to DRS overheadof Release-9, some resource elements (REs) which are not used for theDRSs in Release-9 should be used for RS transmission. However, if arelease-9 UE does not recognize this situation, the release-9 UEinterprets that information (e.g., data) other than the RS istransmitted on the REs and may not accurately receive a downlink signal.Accordingly, if the UE is informed that DRS overhead is increased usingthe TB-to-CW swap flag, the Release-9 UE may recognize that the REpositions where the RS defined in Release-10 are transmitted is null REsand accurately receive a downlink signal.

Alternatively, in case of transmission of rank 1, the TB-to-CW swap flagmay be used as a flag indicating whether transmission of rank 1 issingle-layer beamforming transmission or transmit diversitytransmission.

Hereinafter, various embodiments of the present invention for newlyinterpreting the bit fields of DCI format 2A will be described ingreater detail in addition to the above-described proposals.

Embodiment 1

One embodiment of the present invention for newly interpreting some bitfields of DCI format 2A will now be described.

In the following embodiments, the UE may be informed of whether only oneTB is enabled through explicit signaling or implicitly (that is, throughan MCS value and/or an RV value of the TB).

In Embodiment 1-1, a TB-to-CW swap flag is used as a codeword indicator.

According to definition of the existing DCI 2A format, if only one TB isenabled, the TB-to-CW swap flag is reserved and a TB 1 and a TB 2 aremapped to a codeword 0 (see Table 2). In the present embodiment, amethod of using a TB-to-CW swap flag as an indicator of a codeword, towhich one TB is mapped, if only one TB is enabled is proposed. That is,the TB-to-CW swap flag may be reused as information indicating an indexof a codeword used for single-layer beamforming.

According to the present embodiment, if only one TB is enabled, theTB-to-CW swap flag is not reserved and is set to a 1-bit value. If onlyone TB is enabled and a logical value of the swap flag is a first value,a TB 1 is interpreted to be mapped to a codeword 0 when only the TB 1 isenabled and a TB 2 is interpreted to be mapped to a codeword 1 when onlythe TB 2 is enabled. If only one TB is enabled and a logical value ofthe swap flag is a second value, a TB 1 is interpreted to be mapped to acodeword 1 when only the TB 1 is enabled and a TB 2 is interpreted to bemapped to a codeword 0 when only the TB 2 is enabled.

The first logical value of the swap flag may correspond to 0 or off andthe second logical value of the swap flag may correspond to 1 or on. Thefirst and second logical values may be 1/0 or on/off, but the presentinvention is not limited thereto. That is, alternatively, the firstlogical value may correspond to 1 or on and the second logical value maycorrespond to 0 or off.

The meaning of the first or second logical value is equally applied toother bit fields in the following embodiments of the present invention.That is, a certain bit field having a first or second logical valuemeans that the bit field has a logical value of 1/0 or on/off.

Tables 9 and 10 show the TB-to-CW mapping relationship of Embodiment1-1.

TABLE 9 TB to CW mapping swap flag (one TB enabled) = 0 TB 1 TB 2 CW 0CW 1 enabled disabled TB 1 — disabled enabled — TB 2

TABLE 10 TB to CW mapping swap flag (one TB enabled) = 1 TB 1 TB 2 CW 0CW 1 enabled disabled — TB 1 disabled enabled TB 2 —

In Embodiment 1-2, a TB-to-CW swap flag is used as a codeword indicator.

According to the present embodiment, if only one TB is enabled, theTB-to-CW swap flag is not reserved and is set to a 1-bit value. If onlyone TB is enabled and a logical value of the swap flag is a first value(0 or off), the enabled TB is interpreted to be mapped to a codeword 0.If only one TB is enabled and a logical value of the swap flag is asecond value (1 or on), the enabled TB is interpreted to be mapped to acodeword 1. Tables 11 and 12 show a TB-to-CW mapping relationship ofEmbodiment 1-2.

TABLE 11 TB to CW mapping swap flag (one TB enabled) = 0 TB 1 TB 2 CW 0(enabled) CW 1 (disabled) enabled disabled TB 1 — disabled enabled TB 2—

TABLE 12 TB to CW mapping swap flag (one TB enabled) = 1 TB 1 TB 2 CW 0(disabled) CW 1 (enabled) enabled disabled — TB 1 disabled enabled — TB2

In Embodiment 1-3, a TB-to-CW swap flag is used as a layer indicator.

According to the present embodiment, in the case in which only one TB isenabled and the other TB is disabled, the UE may interpret that channelinformation of a first layer is acquired if the logical value of theswap flag is a first value (0 or off) and interpret that channelinformation of a second layer is acquired if the logical value of theswap flag is a second value (1 or on).

Although the case in which transmission is performed using a transmitdiversity scheme when only one codeword is enabled is described, a2-layer based transmit diversity scheme may be applied. The UE mayacquire information about two channels from a DRS transmitted via eachlayer. In this case, a codeword-to-layer mapping relationship may followthe mapping relationship of Table 4.

Embodiment 1-4 relates to a method of reusing a new data indicator (NDI)or a redundancy version (RV) field of a disabled TB.

As described above, in DCI format 2A, an MCS field, a NDI field and anRV field are defined with respect to a TB. If one TB is enabled and theother TB is disabled, the NDI field or RV field of the disabled TB maybe used for other purposes. A TB may be set to be disabled if the MCSindex value of the TB is 0 or if the MCS index value is 0 and the RVvalue is 1, as described above.

Since a maximum of two layers is used in a dual-layer beamformingscheme, a base station may indicate one layer or antenna port used forsingle antenna port transmission between two layers using a 1-bit fieldof DCI information. For example, as shown in Table 13, a layer used fortransmission is a first layer if the NDI value of the disabled TB is afirst value (or 0) and is a second layer if the NDI value of thedisabled TB is a second value (or 1).

TABLE 13 Indication of antenna port(or layer) for single-antenna port(orlayer) transmission (one TB disabled) New Data Indicator of the disabledTB Antenna port(or layer) 0 1^(st) antenna port(or layer) 1 2^(nd)antenna port(or layer)

Meanwhile, instead of using the NDI field of the disabled TB, a layermay be indicated using an RV field. If the TB is disabled when the MCSindex value of the TB is 1, a first layer may be indicated if the RVfield of the disabled TB is a first value (or 0) and a second layer maybe indicated if the RV field is a second value (or 1).

In Embodiment 1-4-1, a layer number (or an index) to which an enabled TBis mapped may be indicated through the NDI or RV field of the disabledTB. As described with reference to Table 5, the NDI field of DCI format2A has a size of 1 bit and the RV field has a size of 2 bits.Accordingly, a layer used for transmission may be indicated using all orsome of a total of 3 bits for the NDI and RV associated with thedisabled TB.

For example, in a transmission scheme supporting a maximum of two ranks,two layers are used. In this case, it indicates whether a layer used fortransmission is a first layer or a second layer using 1 bit among bitsfor the NDI or RV. For example, in a transmission scheme supporting amaximum of four ranks, four layers are used. In this case, it mayindicate which of first to fourth layers is used for transmission using2 bits among bits for the NDI or RV. For example, in a transmissionscheme supporting a maximum of eight ranks, eight layers are used. Inthis case, it may indicate which of first to fourth layers is used fortransmission using 3 bits among bits for the NDI or RV.

In Embodiment 1-4-2, a layer group number (or an index) to which anenabled TB is mapped may be indicated through the NDI or RV field of thedisabled TB. For example, layers used for transmission are divided intoN groups and it may indicate which of the N layer groups is used fortransmission using M bits among a total of 3 bits for the NDI and RV.

For example, if four layers are divided into two groups, a layer groupindex may be indicated using 1 bit. For example, if eight layers aredivided into four groups, a layer group index may be indicated using 2bits. For example, if eight layers are divided into two groups, a layergroup index may be indicated using 1 bit.

In Embodiment 1-4-3, information indicating whether RS overhead ischanged may be indicated through the NDI or RV field of a disabled TB.As described above, DRS overhead may be changed according to a totaltransmission rank and a DRS according to rank may be defined in 3GPP LTERelease-10, but may not be defined in Release-9. Accordingly, when aRelease-9 UE operates in the Release-10 system, an RS may be transmittedin a state of exceeding RS overhead which may be recognized by theRelease-9 UE. In this case, if an RS is transmitted in an RE region inwhich data is recognized as being transmitted by a Release-9 user, aproblem may occur upon receipt of data from the viewpoint of theRelease-9 UE. Accordingly, it is necessary to inform the Release-9 UE asto whether there is an RE in which data is not transmitted.

Information about whether RS overhead is changed may be indicated using1 bit among bits for the NDI and/or RV of the disabled TB. If an RSoverhead change flag has a first value (e.g., on), the Release-9 UE mayoperate so as not to read data in an RE corresponding to a positionwhere an RS is transmitted in the Release-10 system. In addition, whenan effective coding rate is calculated, the number of subcarriers may becalculated in consideration of increased RS overhead. The effectivecoding rate may be calculated according to a burst size/(the number ofsubcarriers×a modulation order).

In Embodiment 1-4-4, an NDI or RV field of a disabled TB may be used asan indicator indicating whether transmit diversity transmission isperformed.

As described above, if two antenna ports are used, a precodinginformation field of DCI format 2A is not defined. According to theexisting DCI format 2A, a precoding information field is not definedwith respect to 2 transmit antenna ports, but is defined such that anoperation is performed according to a transmit diversity scheme if onecodeword is enabled (that is, in case of rank 1). If DCI format 2A isused for dual-layer beamforming, transmission of rank 1 is not clearlyindicated. That is, it is not indicated whether transmission of rank 1is transmit diversity transmission or beamforming of rank 1.

More specifically, if a precoding information field is not defined, adetermination as to whether a rank-1 beamforming scheme or a rank-2beamforming scheme is used may be made depending on whether twocodewords are enabled. Even in a dual-layer beamforming scheme, transmitdiversity needs to be defined using a basic transmission scheme.However, since the transmit diversity scheme corresponds to transmissionof rank 1, it is difficult to indicate whether a rank-1 beamformingscheme or a transmit diversity scheme is used only depending on whetherone codeword is disabled. Accordingly, if one codeword is disabled, itis necessary to indicate whether a rank-1 beamforming scheme or atransmit diversity scheme is used.

It is possible to define whether a transmit diversity scheme is usedusing the NDI or RV field of the disabled TB. For example, the transmitdiversity scheme is used if the NDI value of the disabled TB is a firstvalue and the rank-1 beamforming scheme is used if the NDI value is asecond value. In case of the transmit diversity scheme, the UE mayperform data demodulation using a cell-specific RS (CRS) or using a DRSfor 2-layer transmission.

Embodiment 1-5 relates to a method of supporting an operation forenabling a Release-9 UE to recognize the position of an RE in which anRS defined in 3GPP LTE Release-10 is transmitted as a null RE. For thismethod, a TB-to-CW swap flag may be used as an indicator indicatingwhether a null RE is present for DRS transmission defined in Release-10.

For example, if the TB-to-CW swap flag is a first value (e.g., 0), itmay indicate that a Release-9 DRS pattern is used. In this case, if a TB1 is enabled and a TB 2 is disabled, transmission is performed via alayer 0 and, if the TB 1 is disabled and the TB 2 is enabled,transmission is performed via a layer 1.

For example, if the TB-to-CW swap flag is a second value (e.g., 1), itmay indicate that a null RE is present while using a Release-9 DRSpattern. In this case, if a TB 1 is enabled and a TB 2 is disabled,transmission is performed via a layer 0 and, if the TB 1 is disabled andthe TB 2 is enabled, transmission is performed via a layer 1.

In Embodiment 1-5, a TB-to-CW mapping relationship may be set as shownin Table 2.

In Embodiment 1-6, a TB-to-CW mapping swap flag field is used as anindicator indicating a transmission scheme.

In the present embodiment, if only one TB is enabled, one of a transmitdiversity scheme or a single-layer beamforming scheme may be indicatedaccording to the value of the TB-to-CW swap flag. For example, if theTB-to-CW swap flag value is a first value (e.g., 0), it indicates thatthe transmit diversity scheme is used. If the TB-to-CW swap flag valueis a second value (e.g., 1), it indicates that the single-layerbeamforming scheme is used. In this case, if a TB 1 is enabled and a TB2 is disabled, transmission is performed via a layer 0 and, if the TB 1is disabled and the TB 2 is enabled, transmission is performed via alayer 1.

In Embodiment 1-7, a precoding information field defined in DCI format2A is newly interpreted for dual-layer beamforming.

In DCI format 2A defined for open-loop spatial multiplexing, theprecoding information field is set to 0 bits in case of 2-antenna porttransmission and is set to 2 bits in case of 4-antenna porttransmission. Since a dual-layer beamforming transmission mode uses amaximum of 2 antenna ports, the precoding information field is notnecessary as described above and the “precoding information” field maybe set to 0 bits or may be defined so as not to be interpreted even whenbits are allocated to the precoding information field in a dual-layerbeamforming transmission mode.

As shown in Table 6, in the existing DCI format 2A, a reserved bit field(e.g., bit fields 2 and 3 if one codeword is enabled and a bit field 3if two codewords are enabled) is present in the precoding informationfield for four antenna ports. The reserved bit field may be defined fordual-layer beamforming.

According to DCI format 2A, as described above, if only one codeword isenabled, it may be interpreted that a single-layer precoding scheme or atransmit diversity scheme is used. Accordingly, if dual-layerbeamforming is applied, there is a need for downlink controlinformation, in order to indicate which scheme is used. Hereinafter,various methods of the present invention for defining some reservedfields of the precoding information field for four antenna ports fordual-layer beamforming will be described.

As shown in Table 14, if only one codeword is enabled, “single layerprecoding” may be explicitly indicated via a predetermined bit value ofthe precoding information field for four antenna ports.

TABLE 14 One codeword: Two codewords: Codeword 0 enabled, Codeword 0enabled, Codeword 1 disabled Codeword 1 enabled Bit field Bit fieldmapped mapped to index Message to index Message 0 4 layers: Transmit 0 2layers: precoder diversity cycling with large delay CDD 1 2 layers:precoder 1 3 layers: precoder cycling with large cycling with largedelay CDD delay CDD 2 Single layer 2 4 layers: precoder precodingcycling with large delay CDD 3 reserved 3 reserved

Alternatively, as shown in Table 15, the precoding information field forfour antenna ports may indicate whether RS overhead is changed. Morespecifically, if a Release-9 UE operates in the 3GPP LTE Release-10system, as described above, RS overhead may be changed. In this case,since the Release-9 UE cannot know an RS position which is additionallydefined with respect to a Release-10 UE, serious performancedeterioration may occur in downlink data demodulation of the Release-9UE. Accordingly, it is necessary to inform the Release-9 UE ofinformation indicating that RS overhead is increased and informationindicating that RS overhead is increased rank-1 precoding and rank-2precoding using a predetermined bit value of the precoding informationfield for four antenna ports of DCI format 2A.

TABLE 15 One codeword: Two codewords: Codeword 0 enabled, Codeword 0enabled, Codeword 1 disabled Codeword 1 enabled Bit field Bit fieldmapped mapped to index Message to index Message 0 4 layers: Transmit 0 2layers: precoder diversity cycling with large delay CDD 1 2 layers:precoder 1 3 layers: precoder cycling with large cycling with largedelay CDD delay CDD 2 Single layer 2 4 layers: precoder precodingcycling with large delay CDD 3 Single layer 3 2 layers: Precodingprecoding (RS without CDD (RS overhead increase) overhead increase)

Alternatively, as shown in Table 16, the precoding information field forfour antenna ports may indicate single layer precoding and indicatewhich of first and second layers (layer 0 or layer 1) corresponds tosingle layer precoding.

TABLE 16 One codeword: Two codewords: Codeword 0 enabled, Codeword 0enabled, Codeword 1 disabled Codeword 1 enabled Bit field Bit fieldmapped mapped to index Message to index Message 0 Transmit Diversity 0 2layers: precoding without CDD 1 Single layer precoding 1 — (layer 0) 2Single layer precoding 2 — (layer 1) 3 — 3 —

In Table 16, a bit field 0 of the precoding information field indicatestransmit diversity if only one codeword is enabled. In this case, the UEmay perform data demodulation using a CRS or a DRS for two-layertransmission.

Alternatively, as shown in Table 17, the precoding information field forfour antenna ports may indicate whether RS overhead is changed and whichof first and second layers (layer 0 or layer 1) corresponds to singlelayer precoding.

TABLE 17 One codeword: Two codewords: Codeword 0 enabled, Codeword 0enabled, Codeword 1 disabled Codeword 1 enabled Bit field Bit fieldmapped mapped to index Message to index Message 0 Single layer precoding0 2 layers: precoding (layer 0) without CDD 1 Single layer precoding 1 2layers: precoding (layer 1) without CDD (RS overhead increase) 2 Singlelayer precoding 2 — (layer 0) (RS overhead increase) 3 Single layerprecoding 3 — (layer 1) (RS overhead increase)

In Table 17, if only one codeword is enabled and the transmit diversityscheme is used, the UE may perform data demodulation using a CRS or aDRS for two-layer transmission.

According to the above-described embodiment of the present invention, aDCI format which can simultaneously support SU-MIMO and MU-MIMO indual-layer beamforming is provided. That is, DCI formats used fordual-layer beamforming and single-layer beamforming may have the samebit field size and switching therebetween may be dynamically performedby transmitting a PDCCH including corrected DCI format 2A proposed bythe present invention.

Embodiment 2

Next, another embodiment of the present invention for newly interpretingbit fields of DCI format 1A or DCI format 1D for dual-layer beamformingwill be described.

Table 18 shows DCI formats 1 and 1A defined in the conventional 3GPP LTEstandard (e.g., 3GPP LTE Release-8). DCI format 1/1A includes controlinformation used for transmission of rank 1, such as a single antenna, asingle stream, transmit diversity transmission.

TABLE 18 DCI format 1 Resource Allocation Header 1 bit Resource blockassignment N bits Modulation and coding scheme 5 bits HARQ processnumber 3 bits (FDD), 4 bits(TDD) New data Indicator 1 bit Redundancyversion 2 bits TPC command for PUCCH 2 bits Downlink Assignment Index 2bits DCI format 1A Flag for format 0/1A differentiation 1 bitLocalized/Distributed VRB assignment Flag 1 bit Resource blockassignment N bits Modulation and coding scheme 5 bits HARQ processnumber 3 bits (FDD1, 4 bits(TDD) New data Indicator 1 bit Redundancyversion 2 bits TPC command for PUCCH 2 bits Downlink Assignment Index 2bits

DCI format 1A is defined for compact scheduling one PDSCH codeword invarious transmission modes and may be used for the transmit diversityscheme. In the present invention, methods for newly interpreting DCIformat 1A for dual-layer beamforming are proposed. If dual-layerbeamforming is defined as a transmission mode (a transmission mode issemi-statically set through higher layer signaling as described above),the UE may interpret some bit fields as having a different meaning fromthe transmit diversity scheme in interpretation of DCI format 1A.

Among fields defined in DCI format 1A, a “flag for format 0/format 1Adifferentiation” is set to 1 bit, the value 0 of the “flag for format0/format 1A differentiation” indicates format 0 and the value 1 of the“flag for format 0/format 1A differentiation” indicates format 1A.Format 1A is used for a random access procedure initiated by a PDCCHorder only when format 1A CRC is scrambled with a C-RNTI.

In addition, among fields defined in DCI format 1A, a“localized/distributed VRB assignment flag” is set to 1 bit, and is setto 0 if the “flag for format 0/format 1A differentiation” is set to 1(that is, in case of format 1A). Otherwise (that is, in case of format0), the value 0 of the “localized/distributed VRB assignment flag”indicates localized VRB assignment and the value 1 of the“localized/distributed VRB assignment flag” indicates distributed VRBassignment.

In Embodiment 2-1 of the present invention, the “flag for format0/format 1A differentiation” is newly interpreted for dual-layerbeamforming. For example, if the logical value of the “flag for format0/format 1A differentiation” is a first value, a transmit diversityscheme is used and, if the logical value of the “flag for format0/format 1A differentiation” is a second value, the single-layerbeamforming scheme is used. As described above, the first value of thelogical value of a certain bit field indicates 0 or off and the secondvalue indicates 1 or on, and vice versa.

Alternatively, if the logical value of the “flag for format 0/format 1Adifferentiation” is a first value, a first layer (layer 0) may beindicated in a DRS pattern and, if the logical value of the “flag forformat 0/format 1A differentiation” is a second value, a second layer(layer 1) may be indicated in a DRS pattern.

Alternatively, if the logical value of the “flag for format 0/format 1Adifferentiation” is a first value, a Release-9 DRS pattern is used and,if the logical value of the “flag for format 0/format 1Adifferentiation” is a second value, null is applied to an RE in which aDRS is additionally transmitted in Release-10 while using a Release-9DRS pattern.

In Embodiment 2-2, the “localized/distributed VRB assignment flag” isnewly interpreted for dual-layer beamforming.

For example, if the logical value of the “localized/distributed VRBassignment flag” is a first value, the transmit diversity scheme is usedand, if “localized/distributed VRB assignment flag” is a second value,the single-layer beamforming scheme is used.

Alternatively, if the logical value of the “localized/distributed VRBassignment flag” is a first value, a first layer (layer 0) may beindicated in a DRS pattern and, if “localized/distributed VRB assignmentflag” is a second value, a second layer (layer 1) may be indicated in aDRS pattern.

Alternatively, if the logical value of the “localized/distributed VRBassignment flag” is a first value, a Release-9 DRS pattern is used and,if “localized/distributed VRB assignment flag” is a second value, nullis applied to an RE in which a DRS is additionally transmitted inRelease-10 while using a Release-9 DRS pattern.

In Embodiment 2-3, 2 bits for the “flag for format 0/format 1Adifferentiation” and the “localized/distributed VRB assignment flag” arenewly interpreted for dual-layer beamforming.

For example, 1 bit of 2 bits for the “flag for format 0/format 1Adifferentiation” and the “localized/distributed VRB assignment flag” mayindicate transmit diversity or single-layer beamforming. If single-layerbeamforming is indicated, the remaining 1 bit may include one of a firstlayer or a second layer in a DRS pattern.

For example, 1 bit of 2 bits for the “flag for format 0/format 1Adifferentiation” and the “localized/distributed VRB assignment flag” mayindicate transmit diversity or single-layer beamforming. If single-layerbeamforming is indicated, the remaining 1 bit may indicate whether nullis applied to an RE in which a DRS is additionally transmitted inRelease-10.

For example, 1 bit of 2 bits for the “flag for format 0/format 1Adifferentiation” and the “localized/distributed VRB assignment flag” mayindicate a first layer (layer 0) in a DRS pattern and, if a logicalvalue is a second layer, a second layer (layer 1) may be indicated. Atthis time, the remaining 1 bit may indicate whether null is applied toan RE in which a DRS is additionally transmitted in Release-10.

Table 19 shows DCI format 1D defined in the existing 3GPP LTE standard(Release-8).

TABLE 19 Localized/Distributed VRB assignment Flag 1 bit Resource BlockAssignment N-bits MCS 5 bits HARQ 3 bits(FDD), 4 bits(TDD) NDI 1 bit RV2 bits TPC command for PUCCH 2 bits DL Assignment Index 0 bit(FDD), 2bits(TDD) TPMI Information for precoding 2 bits(2Tx), 4 bits(4Tx)

DCI format 1D is defined for compact scheduling of one PDSCH codewordhaving precoding and power offset information and may be used forMU-MIMO transmission. In the present invention, methods for newlyinterpreting DCI format 1D for dual-layer beamforming are proposed. Ifdual-layer beamforming is defined as a transmission mode, the UE mayinterpret some bit fields of DCI format 1D as having a different meaningfrom MU-MIMO.

Among the fields defined in DCI format 1D, a “TPMI information forprecoding” field indicates a codebook index used for transmission, has 2bits if the number of antenna ports of a base station is 2, and has 4bits if the number of antenna ports of the base station is 4.

According to Embodiment 2-4 of the present invention, a “TPMIinformation for precoding” field of DCI format 1D may be newlyinterpreted for dual-layer beamforming.

For example, the transmit diversity scheme or the single-layerbeamforming scheme may be indicated using 1 bit of the “TPMI informationfor precoding” field. If the logical value of 1 bit of the “TPMIinformation for precoding” field with 2 bits or 4 bits is a first value,the transmit diversity scheme is indicated and, if the logical value of1 bit of the “TPMI information for precoding” field with 2 bits or 4bits is a second value, the single-layer beamforming scheme isindicated.

Alternatively, using 1 bit of the “TPMI information for precoding”field, a first layer (layer 0) may be indicated in a DRS pattern if thelogical value of the 1 bit is a first value and a second layer (layer 1)may be indicated in a DRS pattern if the logical value of the 1 bit is asecond value.

Alternatively, using 1 bit of the “TPMI information for precoding”field, it may be indicated whether null is applied to an RE in which aDRS is additionally transmitted in Release-10.

Alternatively, between 2 bits of the “TPMI information for precoding”field, 1 bit may indicate the transmit diversity scheme or thesingle-layer beamforming scheme and the remaining 1 bit may indicate afirst layer or a second layer in a DRS pattern.

Alternatively, between 2 bits of the “TPMI information for precoding”field, 1 bit may indicate the transmit diversity scheme or thesingle-layer beamforming scheme and the remaining 1 bit may indicatewhether null is applied to an RE in which a DRS is additionallytransmitted in Release-10.

Alternatively, between 2 bits of the “TPMI information for precoding”field, 1 bit may indicate a first layer or a second layer in a DRSpattern and the remaining 1 bit may indicate whether null is applied toan RE in which a DRS is additionally transmitted in Release-10.

Embodiment 3

Hereinafter, other embodiments of the present invention for defining anew DCI format for dual-layer beamforming will be described.

In dual-layer beamforming transmission, two enabled TBs may be mapped totwo CWs and transmitted via two layers. In addition, the UE may performdata demodulation using channel information acquired through a referencesignal transmitted per layer. In order to transmit two TBs, an MCS, anNDI and an RV are defined with respect to each TB. In addition, aTB-to-CW swap flag for changing a mapping relationship between two TBsand two CWs is defined and robust data transmission may be achievedthrough swapping of the TB-to-CW mapping relationship.

A DRS for dual-layer beamforming may be designed such that layers areorthogonal through a TDM/FDM/CDM scheme. If transmission is performedusing only a single layer, the UE is informed of a reference signalcorresponding to a layer for performing single layer transmission amongreference signals supporting two layers so as to improve datademodulation performance. Accordingly, there is a need for a bit fieldindicating a reference signal used for single-layer beamforming indownlink control information.

When a new (or corrected) DCI format for dual-layer beamforming isdesigned based on DCI format 2A, the following matters may beconsidered.

As described above, dual-layer beamforming has a maximum of rank 2.Since rank is equal to the number of TBs used for transmission, aseparate indicator for a TB is not necessary. In addition, the UE mayconfirm that the TB is disabled if the value of the MCS index of one TBis set to 0 (or the value of the MCS index is set to 0 and the RV valueis set to 1). The UE may implicitly recognize that rank is set to 1 ifone TB is disabled and rank is set to 2 if two TBs are enabled. Inaddition, if a DRS (a precoded UE-specific reference signal) is used perlayer, since a weight matrix used for precoding does not need to beindicated, it is not necessary to define a precoding information in aDCI format in case of dual-layer beamforming using a DRS.

In addition, if a dual-layer beamforming transmission mode is used (thetransmission mode is semi-statically set through higher layersignaling), a DRS for dual-layer beamforming is used and the UE mayreceive data using two layers or a single layer. If dual layers areused, an SU-MIMO operation may be performed and, if a single layer isused, an SU-MIMO or MU-MIMO operation may be performed. The same DCIformat may be used so as not to distinguish between SU-MIMO and MU-MIMOin dual-layer beamforming. That is, in dual-layer beamforming andsingle-layer beamforming, control information is transmitted by a DCIhaving the same bit field size and some bit fields used for dual-layerbeamforming may be interpreted as an indicator for single-layerbeamforming.

In addition, if a dual-layer beamforming transmission mode is used, acompact DCI format may be defined for a UE which receives only a singlelayer.

An example of a new DCI format satisfying the above-describedconsiderations will be described with reference to Table 20. The DCIformat shown in Table 20 includes the same fields as DCI 2A and,hereinafter, a difference between the new DCI format and the existingDCI 2A format will be focused upon.

TABLE 20 Single-layer Dual-layer Beam Beam Resource Block Assignment Nbits N bits TPC command for PUCCH 2 bits 2 bits Downlink AssignmentIndex 2 bits 2 bits HARQ process number 3 bits(FDD), 3 bits(FDD), 4bits(TDD) 4 bits(TDD) Layer Indicator 1 bit 1 bit (One codeword case)Transport Block to codeword swap flag (Two codeword case) Transport MCS5 bits 5 bits block 1 New data Indicator 1 bit 1 bit Redundancy version2 bits 2 bits Transport MCS 5 bits 5 bits block 2 New data Indicator 1bit 1 bit Redundancy version 2 bits 2 bits Precoding information 0 bit 0bit

The DCI format of Table 20 is to provide control information ofsingle-layer beamforming and dual-layer beamforming. In the single-layerbeamforming mode and the dual-layer beamforming mode, resource blockassignment, TPC command for PUCCH, downlink assignment index, HARQprocess number, MCS index, new data indicator and redundancy version fortransport blocks 1 and 2, and precoding information may be defined.These fields substantially have the same meaning as that defined in theexisting DCI format 2A. Among others, the precoding information field isset to 0 bits as described above.

Unlike the existing DCI format 2A, in the DCI format of Table 20, aTB-to-CW swap flag is used for dual-layer beamforming. In case ofsingle-layer beamforming transmission, the TB-to-CW swap flag may beinterpreted as a “layer indicator”.

If two TBs are enabled, the TB-to-CW swap flag may be used asinformation indicating a TB-to-CW mapping relationship and the mappingrelationship may be defined as shown in Table 1.

If one TB is enabled and the other TB is disabled, the enabled TB may bemapped to a codeword 0 as shown in Table 2. In case of single-layerbeamforming in which only one codeword is enabled, the TB-to-CW swapflag is interpreted as a “layer indicator”. If the logical value of thelayer indicator is a first value (0 or off), a first layer (layer X) isindicated, and, if the logical value of the layer indicator is a secondvalue (1 or on), a second layer (layer Y) is indicated. Alternatively,in the logical value of the layer indicator, a first value may indicate1 or on and a second value may indicate 0 or off. The logical value ofthe layer indicator may indicate correspondence between first and secondlayers. The layer indicator may be interpreted as shown in Table 21.

TABLE 21 Transport block to codeword swap flag value Codeword 0 Codeword1 (Layer Indication flag value) (enabled) (disabled) 0 Layer X/Antennaport X 1 Layer Y/Antenna port Y

The “layer indicator” may be referred to as an “antenna port indicator”or a “reference signal (RS) position” and may be interpreted to indicatea layer (or an antenna port) in which an RS is located or alayer/antenna port corresponding to an enabled codeword. The UE mayconfirm, to which layer channel information valid with respect to the UEbelongs, through information acquired from the layer indicator.

According to the new DCI format defined in Table 20, the DCI formats fordual-layer beamforming and single-layer beamforming are set to have thesame size, and dynamic mode adaptation of SU-MIMO and MU-MIMO anddynamic rank adaptation of rank 1 and rank 2 may be implemented.

Next, other embodiments of a new DCI format satisfying theabove-described considerations will be described with reference to Table22. A description of common parts of the DCI formats shown in Tables 20and 22 will be omitted for simplicity.

TABLE 22 Single-layer Dual-layer Beam Beam Resource Block Assignment Nbits N bits TPC command for PUCCH 2 bits 2 bits Downlink AssignmentIndex 2 bits 2 bits HARQ process number 3 bits(FDD), 3 bits(FDD), 4bits(TDD) 4 bits(TDD) Transport Block to codeword swap flag 1 bit 1 bit(Two codeword case) Transport MCS 5 bits 5 bits block 1 New dataIndicator 1 bit 1 bit (Transport block 1 enabled) Layer Indicator(Transport block 1 disabled) Redundancy version 2 bits 2 bits TransportMCS 5 bits 5 bits block 2 New data Indicator 1 bit 1 bit (Transportblock 2 enabled) Layer Indicator (Transport block 2 disabled) Redundancyversion 2 bits 2 bits Precoding information 0 bit 0 bit

In the DCI format of Table 22, a TB-to-CW swap flag is defined,similarly to the existing DCI format 2A. That is, the TB-to-CW swap flagis used for dual-layer beamforming, may be used as informationindicating a TB-to-CW mapping relationship if two codewords are enabled,and the mapping relationship may be defined as shown in Table 1. If oneTB is enabled and the other TB is disabled, the enabled TB may be mappedto a codeword 0 as shown in Table 2.

In the DCI format of Table 22, if the MCS index value of one TB is setto 0 (or the MCS index value is set to 0 and the RV value is set to 1),the TB is enabled and the other TB is disabled, the UE may implicitlyrecognize single-layer beamforming transmission.

An NDI field of a disabled TB may be interpreted as a layer indicator ofan enabled TB. For example, if a TB 1 is enabled and a TB 2 is disabled,the NDI of TB 1 indicates whether data transmitted via the enabled TB 1is new data or retransmitted data and the NDI field of the TB 2 may beinterpreted as a layer indicator (or an antenna port indicator/RSposition) for the TB 1. For example, if the logical value of the NDI ofthe enabled TB is a first value (0 or off), a first layer (layer X) or afirst antenna port (antenna port X) is indicated and, if the logicalvalue of the NDI of the enabled TB is a second value (1 or on), a secondlayer (layer Y) or a second antenna port (antenna port Y) is indicated.Alternatively, the first value of the logical value of the NDI field mayindicate 1 or on, the second value may indicate 0 or off, and thelogical value of the NDI field may indicate correspondence between firstand second layers. The UE may confirm, to which layer channelinformation valid with respect to the UE belongs, through informationacquired from the layer indicator. The layer indicator may beinterpreted as shown in Table 23.

TABLE 23 New data indicator of disabled Codeword 0 Codeword 1 transportblock (enabled) (disabled) 0 Layer X/Antenna port X 1 Layer Y/Antennaport Y

According to the new DCI format defined in Table 22, the DCI formats fordual-layer beamforming and single-layer beamforming are set to have thesame size, and dynamic mode adaptation of SU-MIMO and MU-MIMO anddynamic rank adaptation of rank 1 and rank 2 may be implemented.

Next, other embodiments of a new DCI format satisfying theabove-described considerations will be described with reference to Table24. A description of common parts of the DCI formats shown in Tables 20and 24 will be omitted for simplicity.

TABLE 24 Single-layer Dual-layer Beam Beam Resource Block Assignment Nbits N bits TPC command for PUCCH 2 bits 2 bits Downlink AssignmentIndex 2 bits 2 bits HARQ process number 3 bits(FDD), 3 bits(FDD), 4bits(TDD) 4 bits(TDD)

1 bit

Transport MCS 5 bits 5 bits block 1 New data Indicator 1 bit 1 bit(Transport block 1 enabled) Layer Indicator (Transport block 1 disabled)Redundancy version 2 bits 2 bits Transport MCS 5 bits 5 bits block 2 Newdata Indicator 1 bit 1 bit (Transport block 2 enabled) Layer Indicator(Transport block 2 disabled) Redundancy version 2 bits 2 bits Precodinginformation 1/2/3 bit 1/2/3 bit

In the DCI format of Table 24, a TB-to-CW swap flag is not defined,unlike the existing DCI format 2A. Thus, if two codewords are enabled, acodeword 0 maybe mapped to a TB 1 and a codeword 1 may be mapped to a TB2.

If one TB is enabled and the other TB is disabled, the enabled TB may bemapped to a codeword 0 as shown in Table 2.

In the DCI format of Table 24, if the MCS index value of one TB is setto 0 (or the MCS index value is set to 0 and the RV value is set to 1),the TB is enabled and the other TB is disabled, the UE may implicitlyrecognize single-layer beamforming transmission.

An NDI field of a disabled TB may be interpreted as a layer indicator ofan enabled TB. For example, if a TB 1 is enabled and a TB 2 is disabled,the NDI of TB 1 indicates whether data transmitted via the enabled TB 1is new data or retransmitted data and the NDI field of the TB 2 may beinterpreted as a layer indicator (or an antenna port indicator/RSposition) for the TB 1. For example, if the logical value of the NDI ofthe enabled TB is a first value (0 or off), a first layer (layer X) or afirst antenna port (antenna port X) is indicated and, if the logicalvalue of the NDI of the enabled TB is a second value (1 or on), a secondlayer (layer Y) or a second antenna port (antenna port Y) is indicated.Alternatively, the first value of the logical value of the NDI field mayindicate 1 or on, the second value may indicate 0 or off, and thelogical value of the NDI field may indicate correspondence between firstand second layers. The UE may confirm, to which layer channelinformation valid with respect to the UE belongs, through informationacquired from the layer indicator. The layer indicator may beinterpreted as shown in Table 23.

In the new DCI format defined as shown in Table 24, the precedinginformation field for four antenna ports may be configured as shown inTable 25.

TABLE 25 One codeword: Two codeword: Codeword 0 (/1) enabled, Codeword 0enabled, Codeword 1 (/0) disabled Codeword 1 enabled Bit field Bit fieldmapped mapped to index Message to index Message 0 1 layer in group 1 0 2layers in group 1 1 1 layer in group 2 1 2 layers in group 2 2 1 layerin group 1 2 3 layers (RS overhead Increase) 3 1 layer in group 2 3 4layers (RS overhead Increase)

A layer index to which the enabled codeword is mapped andchange/non-change in RS overhead may be indicated using the precodinginformation shown in Table 25. For example, the index of a layer groupto which the enabled codeword is mapped and how many layers aretransmitted in the layer group may be indicated. In addition to suchinformation, change/non-change in RS overhead may be indicated. In orderto inform the Release-9 UE of information indicating that RS overhead isincreased, information indicating that RS overhead is increased intransmission of rank of may be indicated using a predetermined bit valueof a precoding information field.

According to the new DCI format defined as shown in Table 24, the DCIformats for dual-layer beamforming and single-layer beamforming may beset to have the same size so as to implement dynamic mode adaptation ofSU-MIMO and MU-MIMO and dynamic rank adaptation of rank 1 and rank 2.

The new DCI formats of Tables 20, 22 and 24 may be referred to as DCIformat 2B or DCI format 2C (or other DCI format names) distinguishedfrom the existing DCI formats 2 and 2A and antenna ports X and Y usedfor dual-layer beamforming may be referred to as antenna ports 7 and 8distinguished from the antenna ports defined in the existing LTEstandard.

According to the various embodiments of the present invention, in orderto support dual-layer beamforming, the existing DCI format is newlyinterpreted and a new DCI format distinguished from the existing DCIformat is defined, thereby providing downlink control information to theUE. In particular, it is possible to implicitly determine whether anyone of two TBs is disabled using an MCS field of the TB withoutproviding rank information via an explicit rank indicator in dual-layerbeamforming. In addition, 1-bit information is necessary to indicate alayer (antenna port) used for transmission using a maximum of twolayers. At this time, transmission to two UEs using a single layer maybe supported using a method of indicating a layer using an NDI bit fieldfor a disabled TB between two TBs.

DMRS Based MU-MIMO Transmission Scheme

Hereinafter, the proposal of the present invention for signaltransmission/reception in a wireless communication system supportingMU-MIMO transmission will be described.

The MIMO system refers to a system for improving datatransmission/reception efficiency using multiple transmit antennas andmultiple receive antennas. MIMO technology includes a spatial diversityscheme and a spatial multiplexing scheme. The spatial diversity schemeis suitable for transmitting data to a rapidly moving UE becausetransmission reliability can be increased or cell radius can beincreased through diversity gain. The spatial multiplexing scheme canincrease a data transfer rate without increasing system bandwidth bysimultaneously transmitting different data.

In the MIMO system, each transmit antenna has an independent datachannel. The transmit antenna may mean a virtual antenna or a physicalantenna. The receiver estimates a channel with respect to each transmitantenna and receives data transmitted from each transmit antenna.Channel estimation refers to a process of compensating for signaldistortion generated due to fading and restoring the received signal.Fading refers to a phenomenon wherein a signal level is rapidly changeddue to multipath propagation and time delay in a wireless communicationsystem. For channel estimation, there is a need for a reference signalknown to both a transmitter and a receiver. In addition, the referencesignal may be referred to as an RS or a pilot according to standard.

Among various downlink reference signals, a UE-specific demodulation RS(DMRS) for data demodulation is defined. The DMRS may be called adedicated RS (DRS) as described above. In MU-MIMO transmission, aUE-specific DMRS may be used. Each UE may perform an MU-MIMO operationwithout interference with other UEs using channel information acquiredthrough a precoding based DMRS.

In MU-MIMO transmission supporting multiple layers, DMRS overhead andpositions of DMRSs on RBs may be changed according to transmission rank.If each UE which performs an MU-MIMO operation is not aware of presenceof another UE which performs the MU-MIMO operation, each UEmisrecognizes the DMRSs allocated to the RBs for the another UE as DMRSsallocated for data transmission of each UE and malfunction may be causedupon data demodulation. Accordingly, the present invention proposes amethod of enabling each UE to accurately operate in MU-MIMO transmissionsupporting multiple layers.

FIG. 7 is a diagram illustrating a pattern of a common reference signal(CRS) and a dedicated reference signal (DRS) in a 3GPP LTE system (e.g.,Release-8). The CRS may be called a cell-specific reference signal andthe DRS may be called a UE-specific reference signal.

FIG. 7 is a diagram illustrating REs to which a CRS and a DRS are mappedin a normal CP case. In FIG. 7, a horizontal axis indicates a timedomain (OFDM symbol unit) and a vertical axis indicates a frequencydomain (subcarrier unit). In association with a reference signalpattern, in the normal CP case, 14 OFDM symbols in the time domain and12 subcarriers in the frequency domain become a basic unit of an RB. Inthe extended CP case, 12 OFDM symbols and 12 subcarriers become a basicunit of an RB for a reference signal pattern. Within the time-frequencydomain shown in FIG. 7, a smallest rectangular region corresponds to oneOFDM symbol in the time domain and corresponds to one subcarrier in thefrequency domain.

In FIG. 7, Rp indicates an RE used to transmit a reference signal via ap-th antenna port. For example, R0 to R3 indicate REs to which CRSstransmitted via 0^(th) to 3^(rd) antenna ports are mapped, and R5indicates an RE to which a DRS transmitted via a 5^(th) antenna port ismapped. The CRSs transmitted via the 0^(th) and 1^(st) antenna ports aretransmitted at an interval of six subcarriers on 0^(th), 4^(th), 7^(th)and 11^(th) OFDM symbols (based on one antenna port). CRSs transmittedvia 2^(nd) and 3^(rd) antenna ports are transmitted at an interval ofsix subcarriers on 1^(st) and 8^(th) OFDM symbols (based on one antennaport). DRSs are transmitted at an interval of four subcarriers on3^(rd), 6^(th), 9^(th) and 12^(th) OFDM symbols of every subframe.Accordingly, 12 DRSs are transmitted within two RBs (RB pair), which arecontiguous in terms of time, of one subframe.

A CRS (or a cell-specific reference signal) is used to estimate thechannel of the physical antenna and is commonly transmitted to all UEslocated within the cell. Channel information estimated by a UE through aCRS may be used to demodulate data transmitted using transmissionmethods such as single antenna transmission, transmit diversity,closed-loop spatial multiplexing, open-loop spatial multiplexing, andmulti-user MIMO (MU-MIMO), and may be used to enable a UE to measure achannel and report the channel measurement to a base station. In orderto enhance channel estimation performance through a CRS, the positionsof the CRSs in the subframe may be shifted on a per cell basis to bedifferent from each other. For example, if the RSs are located at aninterval of three subcarriers, the CRS may be arranged on a 3k-thsubcarrier in a certain cell and the CRS may be located on a (3k+1)-thsubcarrier in another cell.

A DRS (or a UE-specific RS) is used for data demodulation and thus maybe referred to as a demodulation reference signal (DMRS). By utilizing apreceding weight used for a specific UE upon MIMO transmission in an RSwithout change, a UE can estimate an equivalent channel, in which theprecoding weight transmitted through each transmit antenna and atransmission channel are combined, when receiving the RS. In addition,the DRS requires orthogonality between transmission layers.

A conventional 3GPP LTE system (e.g., a 3GPP LTE Release-8 system)supports a maximum of 4-transmit (Tx) antenna transmission and defines acell-specific RS for supporting a single Tx antenna, a 2-Tx antenna anda 4-Tx antenna and a UE-specific RS for rank 1 beamforming. Meanwhile,in a 3GPP LTE Release-9 system, one UE may receive transmission of amaximum of rank 2.

Meanwhile, in an LTE-Advanced (LTE-A) system (a 3GPP LTE Release-10system and a system according to the subsequent standard thereof) whichis evolved from the 3GPP LTE system, high order MIMO, multi-celltransmission or advanced multi-user (MU)-MIMO may be considered. Thus, aMIMO scheme for performing transmission of a maximum of rank 8 isapplied to one UE. In the LTE-A system, in order to support efficient RSmanagement and an advanced transmission method, data demodulation basedon a DRS is considered. As described above, DRS based data demodulationrefers to, for example, acquisition of channel information fordemodulating downlink data from a DRS in the case in which a UE receivesthe downlink data from a base station. In addition, the DRS ispreferably set such that downlink transmission by a base station is onlypresent in a scheduled resource block and layer. For example,transmission of a maximum of rank 8 based on a DRS indicates that amaximum of eight DRSs is multiplexed with data for a UE and istransmitted.

In arrangement of the DRS for supporting transmission of a maximum ofrank 8 on radio resources, DRSs for layers may be multiplexed andarranged. Time division multiplexing (TDM) indicates that DRSs for twoor more layers are arranged on different time resources (e.g., OFDMsymbols). Frequency division multiplexing (FDM) indicates that DRSs fortwo or more layers are arranged on different frequency resources (e.g.,subcarriers). Code division multiplexing (CDM) indicates that DRSs fortwo or more layers arranged on the same radio resources are multiplexedusing an orthogonal sequence (or orthogonal covering).

Hereinafter, a DMRS will be described in greater detail. As describedabove, design of a 3GPP LTE-A system supporting a higher uplink/downlinktransfer rate than that of the 3GPP LTE system is being discussed. Inthe 3GPP LTE-A system, downlink MIMO transmission supports a maximum ofrank and data demodulation can be performed based on a UE-specific DMRS.Thus, there is a need for design of a DMRS supporting transmission ofranks 1 to 8. DMRSs for transmission of ranks 1 to 2 of LTE-A may beused for dual-layer beamforming of 3GPP LTE Release-9. Prior todescription of the DMRSs used for 3GPP LTE-A downlink MIMO transmission,DMRSs used for downlink MIMO transmission of the conventional 3GPP LTEsystem (release-8 or release-9) will be described.

Even in the 3GPP LTE system which is the previous version of the 3GPPLTE-A system, downlink MIMO transmission was supported. In downlink MIMOtransmission of the 3GPP LTE release-8 system, single-layer beamformingbased on a precoded DMRS (which may be called a DRS or a UE-specific RS)may be supported. If downlink transmission is performed using theprecoded DMRS, since the precoding weight is included in the channelinformation estimated by a receiver through the precoded MDRS, atransmitter does not need to inform the receiver of information aboutthe precoding weight. As the evolved form of such single-layerbeamforming technology, in downlink MIMO transmission of the 3GPP LTErelease-9 system, dual-layer (or dual-stream) beamforming may besupported. Dual-layer beamforming technology is a MIMO transmissionscheme supporting transmission of a maximum of rank 2 based on aprecoded DMRS.

Hereinafter, DMRS design for an LTE-A system will be described.

For downlink MIMO transmission of the LTE-A system, a precoded RS may beused and RS overhead may be reduced by using the precoded RS. Since aDMRS is precoded by the same precoder as a precoder applied to data, aprecoding matrix is transparent to a UE. Accordingly, only a DMRScorresponding to a layer is transmitted, but separate precodinginformation is not transmitted.

DMRS overhead will now be described. DMRS overhead may be defined as thenumber of REs used for DMRS per RB (e.g., one subframe in a time domain×12 subcarriers in a frequency domain) in each transmission rank.

In transmission of rank 1, 12 REs may be used for a DMRS in one RB. Thisis equal to overhead of a DMRS (antenna port index 5) in 3GPP LTErelease-8.

In transmission of rank 2 or more, a maximum of 24 REs may be used for aDMRS in one RB. In transmission of rank 2 or more, the same RE may beused for a DMRS per antenna port in each rank.

In addition, the same DMRS pattern may be used regardless of subframetype (a TDD or FDD scheme). If the same DMRS pattern may be usedregardless of subframe type, complexity of a UE operation can bereduced.

Hereinafter, details of DMRS design for the LTE-A system will bedescribed from the viewpoint of transmission mode independency, rankindependency, subframe independency and DMRS power boosting.

In transmission of a maximum of eight layers in the LTE-A system, aUE-specific precoded DMRS is supported so as to achieve high spectrumefficiency (or bandwidth efficiency). Since a DMRS is UE-specificallydefined, it is necessary to determine whether the DMRS is optimized foreach transmission mode or the same DMRS is used regardless oftransmission mode. In terms of complexity of a UE operation, the unifiedDMRS capable of performing the same demodulation operation regardless oftransmission mode is advantageous. In addition, in light of jointoptimization of SU-MIMO, MU-MIMO and Cooperative Multi-Point (CoMP)transmission/reception technology, use of the same DMRS regardless oftransmission mode is advantageous in that flexible scheduling istransparently performed with respect to a UE among various transmissionmodes. Accordingly, as long as performance is not significantlyinfluenced, the same DMRS pattern is preferably used regardless oftransmission mode.

Use of the same DMRS pattern regardless of rank may be advantageouslyused in that a UE can perform the same demodulation process in differenttransmission modes such as SU-MIMO, MU-MIMO and CoMPtransmission/reception technologies. Use of the same DMRS patternregardless of rank means that the DMRS pattern (e.g., time-frequencyposition and code) of each layer is the same in all ranks. For example,a channel corresponding to a layer index 1 may be estimated by the samechannel estimator regardless of rank. In other words, use of the sameDMRS pattern regardless of rank means that the DMRS pattern of low rankis a subset of the DMRS pattern of high rank. If the same DMRS patternis used regardless of rank, since a UE can perform data demodulationusing the same operation in all transmission modes, complexity of a UEoperation can be reduced. Accordingly, a fixed DMRS pattern ispreferably used with respect to each layer regardless of rank.

In order to use one DMRS pattern regardless of subframe type (an FDD orTDD scheme) and maintain commonality in an FDM scheme and a TDD scheme,it is necessary to appropriately set the positions of OFDM symbols usedfor DMRS transmission. More specifically, an OFDM symbol used as a guardperiod for a relay backhaul link (a link between a base station and arelay) subframe, a last OFDM symbol used for synchronization channeltransmission in a TDD scheme, etc. may not be used for DMRStransmission. In addition, an OFDM symbol including a cell-specificreference signal (or a CRS) defined in 3GPP LTE Release-8 may not beused for DMRS transmission. This is because, in the case in which powerboosting of the cell-specific reference signal is used (reference signalpower boosting means that power is borrowed from an RE other than an REallocated for a reference signal among REs of one OFDM symbol), DMRStransmit power is decreased and demodulation performance is deterioratedwhen a DMRS is transmitted on the same OFDM symbol as the OFDM symbol onwhich the cell-specific reference signal is transmitted. Accordingly, inthe OFDM symbols to which the guard period of the relay backhaulsubframe and the cell-specific reference signal are allocated, the DMRSis preferably set so as not to be transmitted.

As described above, DMRS overhead may be set to 12 REs in one RB intransmission of rank 1 and to a maximum of 24 REs in transmission ofhigher ranks. However, DMRS transmit power is also considered DMRSoverhead. If DMRSs for a plurality of layers are multiplexed using a CDMscheme, since DMRS transmit power is shared among the plurality oflayers, channel estimation performance may be deteriorated as the numberof layers is increased. Accordingly, DMRS power boosting may beconsidered.

In light of the above description, several methods of using a DMRSpattern are proposed. As described above, in order to reduce complexityof a UE operation and provide flexibility, a method of multiplexingDMRSs for multiple layers using a CDM scheme may be considered.

Pattern-1 to Pattern-4 of FIGS. 8 and 9 correspond to candidates of CDMbased DMRS patterns supporting a high rank.

In order to set a fixed DMRS pattern for each layer regardless of rank,CDM type multiplexing is applicable to a CDM group including 12 REs. InFIGS. 8 and 9, “C” and “D” indicate CDM groups capable of multiplexing amaximum of four layers. The DMRS patterns shown in FIGS. 8 and 9 maysatisfy the above-described transmission mode independency and rankindependency DMRS properties.

Representatively, the DMRS pattern of FIG. 8(a) will be described. 12REs indicated by “C” form one CDM group. In one CDM group, four layersmay be multiplexed using a CDM scheme using Walsh covering. In otherwords, DMRSs for four layers may be arranged on the same RE and DMRSsfor the respective layers may be distinguished (or identified) using CDMresources. An orthogonal cover of (1, 1, 1, 1) is multiplied withrespect to a first layer, an orthogonal cover of (1, −1, 1, −1) ismultiplied with respect to a second layer, an orthogonal cover of (1, 1,−1, −1) is multiplied with respect to a third layer, and an orthogonalcover of (1, −1, −1, 1) is multiplied with respect to a fourth layer.Alternatively, for example, if DMRSs for 3 or less layers aremultiplexed, 3 or less orthogonal covers among four different orthogonalcovers may be selectively used. For example, if DMRSs for two layers aremultiplexed, two DMRSs may be distinguished (or identified) using two(e.g., (1, 1, 1, 1) and (1, −1, 1, −1)) of four different orthogonalcovers.

In the DMRS patterns of FIGS. 8 and 9, DMRS overhead may be changedaccording to transmission rank. As shown in FIGS. 8 and 9, in order tosupport a maximum of eight layers, two CDM groups are used and each CDMgroup may support a maximum of four layers. Accordingly, DMRS overheadmay be differently defined according to the number of CDM groups. Tothis end, two schemes may be considered.

First, in case of rank 1 and rank 2, DMRS overhead of 12 REs may be set,and, in case of ranks 3 to 8, DMRS overhead of 24 REs may be set. Insuch DMRS overhead setting, rank 1 and rank 2 may be defined in one CDMgroup and two CDM groups may be used from rank 3. Accordingly, in caseof rank 3 or more, since robustness in UE mobility may be increased as alarge amount of DMRSs is used, it is possible to ensure good performancewith respect to low rank. However, RS overhead is too high in case ofrank 3 and rank 4.

Alternatively, in case of ranks 1 to 4, DMRS overhead of 12 REs may beset, and, in case of ranks 5 to 8, DMRS overhead of 24 REs may be set.In such DMRS overhead setting, since ranks 1 to 4 may be defined in oneCDM group, RS overhead may be decreased as compared to theabove-described method from the viewpoint of ranks 3 and 4. However, incase of a high Doppler frequency, channel estimation performance may bedecreased as compared to the above-described method.

In the above-described two methods for DMRS overhead setting, sincethere is a trade-off between channel estimation performance and RSoverhead, it is necessary to appropriately set DMRS overhead in light ofchannel estimation performance and RS overhead.

Hereinafter, a MU-MIMO transmission scheme will be described in greaterdetail. As described above, in order to operate MU-MIMO defined in the3GPP LTE (e.g., Release-8) system, each UE may perform data demodulationusing precoding weight information acquired through a control channeland channel information acquired through a cell-specific referencesignal. When MU-MIMO is performed in the LTE-A system and the 3GPP LTERelease-9 system in which DMRSs capable of supporting multiple layersare designed, a base station does not need to inform each UE of aprecoding weight and each UE may perform an MU-MIMO operation withoutmulti-user interference using channel information acquired through theDMRSs. In order to enable the UE to perform accurate operation, it isnecessary to indicate which layer information among channel informationof multiple layers acquired from the DMRSs is for a specific UE.Hereinafter, an MU-MIMO design method for the 3GPP LTE Release-9 systemand the LTE-A system will be described.

In the 3GPP LTE Release-9 system, dual-layer beamforming extended fromsingle-layer beamforming defined in the 3GPP LTE Release-8 system may besupported using a UE-specific precoded DMRS. Thus, since up to twolayers can be supported, SU-MIMO using the precoded DMRS may besupported. In the SU-MIMO scheme using the precoded DMRS, since aprecoder may be optimized in a base station in a manner of beingtransparent to a UE, it is possible to provide better performance ascompared to a codebook based SU-MIMO scheme. In order to provide highersystem throughput in the 3GPP LTE Release-9 system, MU-MIMO needs to besupported by extending the range of dual-layer beamforming from SU-MIMOto MU-MIMO.

With respect to the MU-MIMO scheme based on dual-layer beamforming inthe 3GPP LTE Release-9 system, whether MU-MIMO is supported indual-layer beamforming, orthogonal DMRS or non-orthogonal DMRS,interference cancellation/suppression in a UE, minimization of influenceon the existing standard document, power sharing indicator, etc. may beconsidered. Hereinafter, such considerations will be described indetail. In addition, advantages of optimization of the MU-MIMO schemeusing precoded multi-layer DMRSs will be described.

In the 3GPP LTE Release-8 system, since a spatial division multipleaccess (SDMA) based MU-MIMO scheme using a precoded DMRS is supportedwithout information about an interference channel, system performance isrelatively low and a UE cannot cancel or suppress co-channelinterference. Accordingly, restricted performance was provided becausesystem performance depends on an antenna configuration and a scheduler.In order to enhance an MU-MIMO scheme in the 3GPP LTE Release-9 system,better system performance and robustness are preferably provided byallowing interference cancellation/suppression of a UE (as long as thereis no influence on the existing standard document).

In support of single-user dual-layer beamforming, in order to supportbetter transmission of rank 2, orthogonal DMRSs may be used.Accordingly, if performance gain can be obtained, orthogonal DMRSs whichare already designed for MU-MIMO are preferably maximally used. In orderto improve MU-MIMO performance, a signal-to-interference-plus-noiseratio (SINR) of a UE may be increased by cancelling or suppressingco-channel interference. Accordingly, for better performance, orthogonalDMRSs allowing interference channel estimation are preferably used.

As described above, in an SDMA based MU-MIMO scheme in the 3GPP LTERelease-8 system, since co-channel interference cannot be cancelledand/or suppressed in a UE, performance is restricted to a specificlevel. In order to allow interference cancellation, a modulation andcoding scheme (MCS) of another UE co-scheduled in the same physicalresource block (PRB) and scheduling information such as channel and rankare necessary and thus excessive signaling overhead may be generated. Inaddition, if co-scheduled UEs are not allocated to the same PRB,signaling overhead for interference cancellation may become severe.Since interference suppression requires only interference channelinformation, interference suppression may be preferable to interferencecancellation as a method of enhancing MU-MIMO in the 3GPP LTE Release-9system. If a UE is aware of presence of another UE interfering therewithand a DMRS index associated with RB demodulation of the UE, interferencechannel estimation may be performed using orthogonal DMRSs. Accordingly,interference suppression is supported in 3GPP LTE Release-9 dual-layerMU-MIMO and control information for interference suppression (a DMRSindex and a co-scheduling indicator of the UE) is preferably provided. Adetermination as to whether explicitly indicated control information isnecessary for a UE may be made depending on influence on the existingstandard document.

Among control information associated with interference suppression, if aco-scheduled indicator is explicitly transmitted in each scheduled PRBin order to avoid scheduling restriction, signaling overhead may beincreased. In this case, a CDM based DMRS may solve such a problem. Thatis, if the CDM based DMRS is used, the UE can detect co-channelinterference with orthogonal DMRSs through energy detection in eachscheduled PRB. In addition, if there is another interfering UE, aninterference signal can be suppressed in each scheduled PRB.Accordingly, a DMRS indicator using a CDM based orthogonal DMRS canminimize influence on the standard document supporting MU-MIMO. If suchapproach is applied to SU-MIMO transmission of rank 1, a common PDCCHmay be used for SU-MIMO and MU-MIMO.

In the 3GPP LTE Release-8 system, since a CRS is cell-specificallyprovided, power sharing information may be required for demodulation ifquadrature amplitude modulation (QAM) is used. However, if a UE-specificRS is used (since the UE-specific RS is not shared with another UE),power sharing information is implicitly included in an RS. In this case,a UE may estimate that an RE on which data is transmitted and an RE onwhich a UE-specific RS is transmitted have the same power level.Accordingly, a determination as to whether DMRS power boosting issupported may be made only in order to enhance channel estimationperformance and the same is true in case of SU-MIMO. Accordingly, inorder to support MU-MIMO, a power sharing indicator may not bespecified.

When considering DMRS based MIMO transmission, as described above, DMRSoverhead may be differently set according to transmission rank. Forexample, DMRSs supporting ranks 1 to 2 have constant overhead, but DMRSssupporting rank 3 or more have increased overhead. Alternatively, DMRSssupporting ranks 1 to 4 have constant overhead, but DMRSs supportingrank 5 or more have increased overhead.

If overhead of DMRSs of ranks 1 to 2 are constantly maintained, DMRSoverhead may be constantly maintained in data transmission through DMRSbased dual-layer beamforming of the 3GPP LTE Release-9 system. Whensupporting MU-MIMO in the 3GPP LTE Release-9 system, a maximum of twoUEs may be multiplexed (each UE performs single-layer transmission), abase station may maintain the positions of the DMRSs on the RB andconstant DMRS overhead even in MU-MIMO because DMRSs of rank 2 aretransmitted in DMRS based MIMO transmission. The base station indicatesa specific layer between two layers to a UE which performs MU-MIMOtransmission such that the UE acquires orthogonal channels. If DMRSoverhead or the positions of the DMRSs on the RB are not changed,MU-MIMO may be performed using a layer indicator.

Meanwhile, with respect to rank which is a criterion for changing DMRSoverhead and the positions of the DMRSs on the RB, if maximum ranksupporting MU-MIMO is determined in a range exceeding the criterion, aserious problem may be generated in data demodulation of a UE whichperforms MU-MIMO. For example, in case of ranks 1 to 2, DMRSs aremultiplexed using a CDM group “C” of FIG. 8(a) (that is, using 12 REs inone RB) and, in case of ranks 3 or more, DMRSs are multiplexed usingboth CDM groups “C” and “D” of FIG. 8(a) (that is, using 24 REs in oneRB). In this case, it is assumed that DMRSs are additionally arranged atpositions of DMRSs supporting ranks 1 to 2 on the RB, for transmissionof rank 3 or more. At this time, if it is assumed that three UEs whichreceive single-layer transmission are multiplexed so as to perform anMU-MIMO operation, a base station performs data transmission using DMRSssupporting rank 3. UEs (first and second UEs) to which a first layer anda second layer are allocated acquire channels from DMRSs of ranks 1 to 2and a UE (a third UE) to which a third layer is allocated acquires achannel from the DMRS which is additionally allocated for DMRS of rank3. As described above, if the layer allocated to each UE is indicatedusing a layer indicator, the UEs (the first and second UEs) to which thefirst layer and the second layer are allocated may not recognizepresence of the third layer. At this time, since the first and secondUEs misrecognize that the REs of the added DMRSs are used to demodulatedata, not DMRSs, a serious problem may be generated in data demodulationof the first and second UEs. Accordingly, if DMRS overhead and thepositions of the DMRSs on the RB are changed according to rank, it isnecessary to inform the UE which performs MU-MIMO of maximum rank.

Embodiment 4

Hereinafter, embodiments of the present invention for DMRS-based MU-MIMOtransmission will be described in detail.

In the present invention, a rank indicator is used in order to inform aUE which performs MU-MIMO of maximum rank. In SU-MIMO, the rankindicator may indicate currently transmitted rank. In MU-MIMO, the rankindicator may be used to indicate total rank in which multiple users aremultiplexed or the number of layers allocated to a specific UE.

More specifically, the rank indicator may be used to indicate totaltransmission rank of UEs which are multiplexed for MU-MIMO. For example,if there are M (M≥2) UEs which perform transmission of rank N (N≥1), abase station performs transmission of a total of K (K=N*M) ranks. Atthis time, the base station informs the UE that transmission of thetotal of K ranks is performed and a DMRS pattern supporting rank K isused through the rank indicator and the UE recognizes that the DMRSsupporting rank K is used. According to such a method, even when theDMRS pattern or overhead is changed according to transmission rank, theUE can distinguish (or identify) between an RE used for DMRS and an REused for data. The UE can acquire channels of K layers through the DMRSsof rank K and confirm which layer has channel information valid withrespect to the UE through information acquired from the layer indicator.

For example, it is assumed that one UE supports transmission of rank 1if total transmission rank is 3. It is assumed that a base stationtransmits DMRSs through 12 REs in one RB up to total transmission rankof 2 and transmits DMRSs through 24 REs in one RB in total transmissionrank of 3 or more. If information about total transmission rank is notknown to a UE which performs MU-MIMO, the UE may misrecognize that datais transmitted at positions other than DMRS positions (12 RE positionsin one RB, for example, positions of “C” of FIG. 8(a)) for transmissionof rank 1. Accordingly, since the UE performs data demodulation on theassumption that data is transmitted at RE positions where DMRSs foranother UE are transmitted (12 additional RE positions in one RB, forexample, the positions of “D” of FIG. 8(a)), data cannot be accuratelydemodulated. In contrast, as described above, if information about totaltransmission rank is known to the UE, the UE may be aware of REpositions to which DMRSs valid with respect to another UE are allocatedin addition to the positions of DMRSs valid with respect to the UE andmay recognize that data is not transmitted at those REs. Accordingly,the UE can distinguish between an RE on which DMRS is transmitted and anRE on which data is allocated and accurately perform data demodulation.

If only transmission of a maximum of rank 1 is allowed with respect toeach UE which performs an MU-MIMO operation, the UE may acquire only thechannel of the layer indicated by a layer indicator.

Meanwhile, if transmission of a maximum of rank P (P≥2) is allowed withrespect to each UE which performs an MU-MIMO operation andmulti-codeword transmission is allowed with respect to each UE, the UEmay acquire channels of P layers from the layer indicated by the layerindicator in ascending order. For example, if data transmission using amaximum of two layers is allowed, channels of a layer indicated by alayer indicator (e.g., a layer index 1) and a next layer (e.g., a layerindex 2) thereof may be acquired. Meanwhile, if single-layertransmission is performed in a state of allowing transmission of amaximum of two layers, the UE may perceive transmission of rank 1through other information. For example, the UE may implicitly confirmwhether a codeword is enabled from MCS information of each code andacquire the number of transmission layers. For example, the UE mayrecognize transmission of rank 2 if two codewords are enabled and mayrecognize transmission of rank 1 if only one codeword is enabled.

If transmission of a maximum of rank P (P≥2) is allowed with respect toeach UE which performs an MU-MIMO operation and single-codewordtransmission is allowed with respect to each UE, the UE may acquirechannels of P layers from the layer indicated by the layer indicator inascending order.

In summary, it is possible to provide total transmission rank,information about layers valid with respect to each UE and rankinformation of each UE to UEs which perform an MU-MIMO operation throughthe same resources, to distinguish between the positions of DMRSs anddata on the RB, and to acquire channel information valid with respect tothe UE.

In various embodiments of the present invention, the following settingsare applied in association with the rank indicator.

First, up to a maximum of rank 8 may be set to be indicated using a3-bit rank indicator.

Second, in indication of total rank multiplexed for MU-MIMO using a2-bit rank indicator, up to a maximum of rank 4 may be set to beindicated.

Third, a 1-bit indicator indicating DMRS overhead or position change maybe used without setting a rank indicator. The 1-bit indicator mayindicate whether DMRS overhead is increased (or additional DMRSpositions) using on/off.

In support of the above-described MU-MIMO operation, a codeword-to-layermapping relationship needs to be newly defined from the viewpoint of thebase station and the UE. This will be described with reference to Tables26 to 37.

Tables 26 to 31 show codeword-to-layer mapping in the case in whichtransmission of a maximum of rank P (P≥2) is allowed with respect toeach UE which performs an MU-MIMO operation and transmission of multiplecodewords (a maximum of two codewords) is allowed with respect to eachUE. Tables 26 to 29 show codeword-to-layer mapping in case of maximumtransmission ranks 1, 2, 3 and 4 from the viewpoint of the base station.Tables 30 and 31 show codeword-to-layer mapping in case of maximumreception ranks 1 and 2 from the viewpoint of the UE.

TABLE 26 Codeword to Layer Max Tx mapping Rank Codeword Layer 1 1 (for1^(st) UE) 1

TABLE 27 Codeword to Layer Max Tx mapping Rank Codeword Layer 2 1 (for1^(st) UE) 1 1 (for 2^(nd) UE) 2

TABLE 28 Codeword to Layer Max Tx mapping Rank Codeword Layer 3 1 (for1^(st) UE) 1 (case 1) 1 (for 2^(nd) UE) 2 1 (for 3^(rd) UE) 3 3 1 (for1^(st) UE) 1 (case 2) 1 (for 2^(nd) UE) 2 2 (for 2^(nd) UE) 3

TABLE 29 Codeword to Layer Max Tx mapping Rank Codeword Layer 4 1 (for1^(st) UE) 1 (case 1) 1 (for 2^(nd) UE) 2 1 (for 3^(rd) UE) 3 1 (for4^(th) UE) 4 4 1 (for 1^(st) UE) 1 (case 2) 1 (for 2^(nd) UE) 2 1 (for3^(rd) UE) 3 2 (for 3^(rd) UE) 4 4 1 (for 1^(st) UE) 1 (case 3) 2 (for1^(st) UE) 2 1 (for 2^(nd) UE) 3 2 (for 2^(nd) UE) 4

TABLE 30 Codeword to Layer Max Rx mapping Rank Codeword Layer 1 1 1

TABLE 31 Codeword to Layer Max Rx mapping Rank Codeword Layer 2 1 1 2 2

Tables 32 to 37 show codeword-to-layer mapping in the case in whichtransmission of a maximum of rank P (P≥2) is allowed with respect toeach UE which performs an MU-MIMO operation and transmission of singlecodeword (one codeword) is allowed with respect to each UE. Tables 32 to35 show codeword-to-layer mapping in case of maximum transmission ranks1, 2, 3 and 4 from the viewpoint of the base station. Tables 36 and 37show codeword-to-layer mapping in case of maximum reception ranks 1 and2 from the viewpoint of the UE.

TABLE 32 Codeword to Layer Max Tx mapping Rank Codeword Layer 1 1 (for1^(st) UE) 1

TABLE 33 Codeword to Layer Max Tx mapping Rank Codeword Layer 2 1 (for1^(st) UE) 1 1 (for 2^(nd) UE) 2

TABLE 34 Codeword to Layer Max Tx mapping Rank Codeword Layer 3 1 (for1^(st) UE) 1 (case 1) 1 (for 2^(nd) UE) 2 1 (for 3^(rd) UE) 3 3 1 (for1^(st) UE) 1 (case 2) 1 (for 2^(nd) UE) 2 3

TABLE 35 Codeword to Layer Max Tx mapping Rank Codeword Layer 4 1 (for1^(st) UE) 1 (case 1) 1 (for 2^(nd) UE) 2 1 (for 3^(rd) UE) 3 1 (for4^(th) UE) 4 4 1 (for 1^(st) UE) 1 (case 2) 1 (for 2^(nd) UE) 2 1 (for3^(rd) UE) 3 4 4 1 (for 1^(st) UE) 1 (case 3) 2 2 (for 2^(nd) UE) 3 4

TABLE 36 Codeword to Layer Max Rx mapping Rank Codeword Layer 1 1 1

TABLE 37 Codeword to Layer Max Rx mapping Rank Codeword Layer 2 1 1 2

The codeword-to-layer mapping relationship in case of maximum receptionrank 2 may be used not only for initial transmission but also forretransmission in MU-MIMO transmission from the viewpoint of the UEshown in Tables 26 and 37.

Control Information Simultaneously Supporting SU-MIMO and MU-MIMO

In the above-described various embodiments of the present invention, ina system supporting dual-layer beamforming based on DMRS, a method ofindicating a layer to be read for data demodulation among DMRSs used fortransmission is proposed. If dual-layer beamforming transmission isperformed, DMRSs may be transmitted through REs in one RB and DMRSs foreach layer may be distinguished (or identified) using two orthogonalcover codes (OCCs). That is, two OCCs may be used as orthogonalresources for distinguishing (or identifying) DMRSs. In addition, as amethod of indicating valid DMRSs among transmitted DMRS resources whenMU-MIMO transmission is performed using dual layers, an NDI bit of adisabled codeword may be used.

A DMRS pattern will be described with reference to FIG. 8(a). In FIG.8(a), DMRSs may be mapped to 24 REs in one RB. In FIG. 8(a), the 12 REscorresponding to the CDM group “C” and the 12 REs corresponding to theCDM group “D” are distinguished (or identified) in the time/frequencydomain. In groups “C” and “D”, four different DMRSs may be distinguished(or identified) using orthogonal code resources. In FIG. 8(a), fourdifferent DMRSs are distinguished (or identified) using four orthogonalWalsh codes. In FIG. 8(a), four different DMRSs are distinguished(identified) using four orthogonal Walsh codes.

Alternatively, one DMRS CDM group may be divided into two subgroups. Thetwo subgroups are quasi-orthogonal to each other. Two layers (or antennaports) may be distinguished (or identified) using fully orthogonalsequences within one subgroup. Two fully orthogonal sequences may be {1,1, 1, 1} and {1, −1, 1, −1}, for example. In other words, DMRSsdistinguished (or identified) by the two fully orthogonal sequences areDMRSs for different antenna ports (e.g., {1, 1, 1, 1} code resources areused with respect to an antenna port X and {1, −1, 1, −1} is used withrespect to an antenna port Y), two DMRSs are distinguished (oridentified) using quasi-orthogonal code resources with respect to therespective antenna ports (e.g., two DMRSs which are distinguished (oridentified) are present with respect to the antenna port X and two DMRSswhich are distinguished (or identified) are present with respect to theantenna port Y). In summary, on one DMRS CDM group (e.g., “C”) composedof 12 REs, a total of four different DMRSs may be distinguished (oridentified) by code resources. Meanwhile, the CDM groups “C” and “D” maybe called group 1 and group 2, respectively. As resources fordistinguishing between DMRSs, since two groups may be used and fourorthogonal resources may be used per group, a total of eight orthogonalresources is secured.

It is assumed that 12 REs are used for DMRS transmission in ranks 1 and2 and 24 REs are used for DMRS transmission in rank 3 or more. Whileonly one group (that is, “C”) is used for DMRS transmission up to rank2, two groups (that is, “C” and “D”) are used for DMRS transmission inrank 3 or more. For example, if DMRSs are transmitted to first to fourthlayers in transmission of rank 4, DMRSs for any two layers (e.g., firstand second layers) are mapped to one group (e.g., “C”) and aredistinguished (or identified) using OCCs and DMRSs for the remaining twolayers (e.g., third and fourth layers) are mapped to another group(e.g., “D”) and are distinguished (or identified) using OCCs.

If MU-MIMO transmission using a maximum of four layers is performed fromthe viewpoint of a transmitter, DMRSs for distinguishing (oridentifying) four layers are used. At this time, each receiver needs toacquire information about a layer corresponding thereto. As describedabove, according to the method of indicating the layer using an NDI bit(1 bit) of a disabled codeword, since up to two UEs each using a singlelayer can be supported, there is a need for a method of indicatingextended layers.

First, a CDM group indication bit may be defined. For example, the UEmay be informed of which of the first and second CDM groups (e.g., “C”or “D” of FIG. 8(a)) the layer belongs to through a 1-bit indicator. Onegroup is composed of 12 REs and a maximum of four DMRSs may bemultiplexed (that is, distinguished) using a CDM scheme and transmittedvia each group. In case of MU-MIMO transmission of a maximum of fourlayers, two DMRSs may be multiplexed using two orthogonal codes using aCDM scheme in one group. If a group is determined through a 1-bit groupindicator, one of the two orthogonal codes may be determined using anNDI (1 bit) of a disabled codeword. In addition, if a determination asto which group is used is made using a 1-bit group indicator and twocodewords are enabled, two orthogonal codes are used and thus theorthogonal codes do not need to be indicated. By defining a 1-bit groupindicator, a signaling method capable of efficiently supporting MU-MIMOtransmission with a maximum of four layers may be provided.

Hereinafter, a method of supporting SU-MIMO and MU-MIMO transmissionusing the same control information format in a system which uses twocodewords and supports a maximum of rank N (N≤8) will be described.Matters to be included in such control information will be describedfirst and a method of configuring a detailed control information formatwill then be described.

(1) As control information for MIMO transmission, rank and precodinginformation are basically necessary.

(2) Rank information indicates the number of virtual antennas (orstreams or layers) used for transmission. When data is demodulated usinga MIMO receiver, the number of layers to be subjected to datademodulation is determined based on rank information. From the viewpointof a single user, rank information may be referred to as “the number oflayers to be received by one user”. If MU-MIMO is applied, a transmittermay transmit data to a plurality of receivers (several users) usingmultiple layers. Accordingly, from the viewpoint of multiple users (orfrom the viewpoint of an MU-MIMO transmitter), rank information may bereferred to as “the number of all layers used for transmission”.

(3) Precoding information refers to information about a precoding weightused for signal transmission by a transmitter. In order to reduce thesize of the indication bit for the precoding weight, a method ofdefining a codebook type precoding weight in advance and reporting acodebook index corresponding to the precoding information used fortransmission may be considered. A receiver may perform data demodulationby coupling the information about the precoding weight indicated by thetransmitter and channel information acquired through a reference signal.Meanwhile, a dedicated reference signal (or a demodulation referencesignal) is precoded by the same precoder as transmitted data.Accordingly, if a dedicated reference signal for a certain user is used,information about the precoding weight is not separately signaled.

(4) In a system which uses a dedicated reference signal, only rankinformation may be basically indicated for MIMO transmission. Forexample, in a system for performing transmission of a maximum of rank 2,a 1-bit rank indicator is necessary to indicate rank. A 2-bit rankindicator is necessary in a system for performing transmission of amaximum of rank 4 and a 3-bit rank indicator is necessary in a systemfor performing transmission of a maximum of rank 8.

(5) The following matters may be considered in multi-codeword (MCW)transmission. Multi-antenna transmission may support transmission usinga single layer and transmission using multiple layers. When performingtransmission using multiple layers, different MCSs are applicable to thelayers. Transmission for applying multiple MCSs to multiple layers maybe referred to as MCW transmission. If MCSs are applied to multiplelayers, signaling overhead can be increased and thus some of themultiple layers may be designed to have the same MCS. For example, in asystem for transmitting two layers, the respective layers may havedifferent MCSs. In a system for transmitting three layers, any one layermay be allocated an MCS for the layer and the remaining two layers maybe allocated an MCS suitable for the channel status of the two layers.The two layers may be designed to have the same MCS. Meanwhile, a bitindicating an MCS may include MCS level and codeword disableinformation.

(6) In a system for transmitting two codewords, when two MCS indicatorsare allocated to a control channel for transmission (each MCS indicatorincludes indication information of codeword disability) and any one MCSindicator indicates codeword disability, a receiver may recognize thatone codeword is not transmitted but only another codeword istransmitted. In addition, if the two MCS indicators indicate certain MCSlevels, the receiver may recognize that two codewords are transmitted.

(7) In the case where MCW transmission is performed in a systemsupporting transmission of a maximum of rank 2, an MCS indicator foreach layer may be used. 1 bit may be allocated as an indicator fortransmission rank. However, in transmission of a maximum of rank 2, evenwhen an indicator for transmission rank is not allocated, adetermination as to whether single-layer transmission or 2-layertransmission is performed is made depending on whether a codeword isenabled. If each codeword is transmitted via each layer, transmissionrank can be conformed depending on which codeword is enabled. Inparticular, in the case in which transmission of a maximum of rank 2 isperformed in a system having two codewords, only one codeword is enabledin transmission of rank 1 and two codewords are enabled in transmissionof rank 2. Accordingly, in a system in which each codeword istransmitted via each layer while supporting multiple codewords, sincetransmission rank information can be acquired via information includedin an MCS indicator, rank information may not be separately signaled.

(8) In the case in which MIMO transmission is performed using adedicated reference signal (DRS or DMRS), orthogonal resources(distinguishable resources) for the reference signal are equal to thenumber of layers used for MIMO transmission. Orthogonal resources fordistinguishing (or identifying) reference signals may be code resources,frequency resources and/or time resources. For example, in case oftransmission of rank 2, two orthogonal resources are required for adedicated reference signal. However, in MU-MIMO, orthogonal resourcescorresponding in number to the number of layers transmitted to multipleusers are required for the dedicated reference signal. For example, inthe case in which transmission of rank 2 is performed with respect totwo users, a total of four orthogonal resources are used for thededicated reference signal.

(9) In order to enable a receiver to demodulate signals in an MU-MIMOsystem which uses a dedicated reference signal, it is necessary todistinguishably (identifiably) indicate reference signals of multipleusers. For example, when signals are simultaneously transmitted to twousers for performing transmission of rank 1, two orthogonal resourcesare used. If the order of the two orthogonal resources is set, it isnecessary to inform each user of the order of orthogonal resourcescorresponding to the signal thereof.

If the DMRS group indication bit, the rank indication bit used forSU-MIMO transmission and the rank indication bit indicating all layersused for MU-MIMO transmission are all defined, clear control informationfor MIMO transmission is provided, but signaling overhead issignificantly increased. In the present invention, an efficient controlinformation signaling method capable of reducing signaling overheadwhile simultaneously supporting SU-MIMO and MU-MIMO is proposed asfollows.

Embodiment 5

The present embodiment relates to a method of indicating rank and layerused for transmission with respect to transmission of rank 1 and rank 2and configuring control information indicating only transmission rankinformation.

For example, in MU-MIMO transmission, it is assumed that each receivermay receive a maximum of 2 layers and a transmitter may transmit amaximum of four layers. In this case, there is a need for an indicationbit for distinguishing (or identifying) four layers for UEs whichparticipate in MU-MIMO. In the case of applying the above-describedindication method for MIMO transmission, control information may beprovided such that a receiver recognizes statuses A to D of Table 38.

TABLE 38 Group 1 Group 2 OCC #1 OCC #2 OCC #1 OCC #2 A 2-layers 2-layersB 1-layer 1-layer 2-layers C 2-layers 1-layer 1-layer D 1-layer 1-layer1-layer 1-layer E 1-layer 2-layers 1-layer

Table 39 shows an example of a codeword-to-layer mapping rule oftransmission of a maximum of eight layers.

TABLE 39 CW to Layer mapping rule (codeword number) - (layer number) #of layers One codeword Two Codeword 1 (1) - (1) X 2 (1) - (1, 2) (1) -(1) (2) - (2) 3 (1) - (1, 2, 3) (1) - (1) (2) - (2, 3) 4 (1) - (1, 2, 3,4) (1) - (1, 2) (2) - (3, 4) 5 (1) - (1, 2) (2) - (3, 4, 5) 6 (1) - (1,2, 3) (2) - (4, 5, 6) 7 (1) - (1, 2, 3) (2) - (4, 5, 6, 7) 8 (1) - (1,2, 3, 4) (2) - (5, 6, 7, 8)

In the a codeword-to-layer mapping rule shown in Table 39, data may betransmitted with rank 1, 2, 3 or 4 if one codeword is enabled and datamay be transmitted with rank 2, 3, 4, 5, 6, 7 or 8 if two codewords areenabled. Accordingly, definition of transmission rank according to thenumber of enabled codewords is expressed as shown in Table 40.

TABLE 40 One codeword: Two codeword: Codeword 0 (/1) enabled, Codeword 0enabled, Codeword 1 (/0) disabled Codeword 1 enabled Bit field Bit fieldmapped mapped to index Message to index Message 0 1 layer 0 2 Layers 1 2Layers 1 3 Layers 2 3 Layers 2 4 Layers 3 4 Layers 3 5 Layers 4 4 6Layers 5 5 7 Layers 6 6 8 Layers 7 7

Table 40 shows precoding information supporting eight antenna ports. Oneto eight layer(s) of Table 40 indicate the number of layers used totransmit data from the viewpoint of a single user. As shown in Table 40,a minimum of 3 bits is required to indicate rank information. In thecase in which 3 bits are used, four fields may be reserved if onecodeword is enabled and one field may be reserved if two codewords areenabled.

In the case in which a rank indicator is used for transmission of up torank 4, 2 bits may be used as a rank indicator. The rank indicator of 2bits may be configured as shown in Table 41. Table 41 shows precodinginformation supporting four antenna ports.

TABLE 41 One codeword: Two codeword: Codeword 0 (/1) enabled, Codeword 0enabled, Codeword 1 (/0) disabled Codeword 1 enabled Bit field Bit fieldmapped mapped to index Message to index Message 0 1 layer 0 2 Layers 1 2Layers 1 3 Layers 2 2 4 Layers 3 3

As shown in Table 41, one field may be reserved if one codeword isenabled and one field may be reserved if two codewords are enabled.

Meanwhile, in the case in which a rank indicator is used fortransmission of up to rank 2, 1 bit may be used as a rank indicator. Therank indicator of 1 bit may be configured as shown in Table 42. Table 42shows precoding information supporting two antenna ports.

TABLE 42 One codeword: Two codeword: Codeword 0 (/1) enabled, Codeword 0enabled, Codeword 1 (/0) disabled Codeword 1 enabled Bit field Bit fieldmapped mapped to index Message to index Message 0 1 layer 0 2 Layers

As shown in Table 42, in the case in which transmission is performedthrough two antenna ports, it is recognized that transmission of rank 1is performed if one codeword is enabled and transmission of rank 2 isperformed if two codewords are enabled. Accordingly, it is possible toaccurately perform an operation even when a separate bit indicating rankis not defined.

Next, a method of defining a layer indication in a precoding informationfield is proposed as follows.

In the case in which MU-MIMO is applied in transmission of ranks 1 and 2and SU-MIMO is applied in transmission of rank 3 or more, the precodinginformation field for eight antenna ports may be configured as shown inTable 43.

TABLE 43 One codeword: Two codeword: Codeword 0 (/1) enabled, Codeword 0enabled, Codeword 1 (/0) disabled Codeword 1 enabled Bit field Bit fieldmapped mapped to index Message to index Message 0 1 layer in 1^(st)group 0 2 Layers in 1^(st) group 1 1 layer in 2^(nd) group 1 2 Layers in2^(nd) group 2 2 Layers 2 3 Layers 3 3 Layers 3 4 Layers 4 4 Layers 4 5Layers 5 5 6 Layers 6 6 7 Layers 7 7 8 Layers

Meanwhile, when one codeword is enabled, MIMO transmission of rank 2, 3or 4 is possible. When one codeword is enabled, it is possible toindicate a group, to which two layers are allocated, in order to enableMU-MIMO transmission in transmission of rank 2. In this case, theprecoding information for eight antenna ports may be configured as shownin Table 44.

TABLE 44 One codeword: Two codeword: Codeword 0 (/1) enabled, Codeword 0enabled, Codeword 1 (/0) disabled Codeword 1 enabled Bit field Bit fieldmapped mapped to index Message to index Message 0 1 layer in 1^(st)group 0 2 Layers in 1^(st) group 1 1 layer in 2^(nd) group 1 2 Layers in2^(nd) group 2 2 Layers in 1^(st) group 2 3 Layers 3 2 Layers in 2^(nd)group 3 4 Layers 4 3 Layers 4 5 Layers 5 4 Layers 5 6 Layers 6 6 7Layers 7 7 8 Layers

In the case in which a UE demodulates data via DMRSs, it is assumed thata maximum number of REs is used for DMRSs. For example, when a DMRS CDMgroup 1 is indicated with respect to an arbitrary UE, data demodulationmay be performed in consideration of an RE position (e.g., a DMRS CDMgroup 2), to which DMRSs for another UE are mapped, other than an RE fordata and a reference signal. A coding rate of transmitted informationmay be calculated in consideration of the maximum number of REs of DMRSsand the information may be encoded and decoded according to the codingrate. With respect to a UE belonging to the DMRS CDM group 1,encoding/decoding may be performed in consideration of the number of REsfor transmission of rank 1 or 2 (e.g., in the case in which a total of24 REs is used for DMRS transmission, only 12 REs are used whentransmission of rank 1 or 2 is performed).

Meanwhile, in the case in which MU-MIMO is applied in transmission ofranks 1 and 2 and SU-MIMO is applied in transmission of rank 3 or more,the precoding information field for four antenna ports may be configuredas shown in Table 45.

TABLE 45 One codeword: Two codeword: Codeword 0 (/1) enabled, Codeword 0enabled, Codeword 1 (/0) disabled Codeword 1 enabled Bit field Bit fieldmapped mapped to index Message to index Message 0 1 layer in 1^(st)group 0 2 Layers in 1^(st) group 1 1 layer in 2^(nd) group 1 2 Layers in2^(nd) group 2 2 Layers 2 3 Layers 3 3 4 Layers

Meanwhile, when one codeword is enabled, it is possible to indicate agroup, to which two layers are allocated, in order to enable MU-MIMOtransmission in transmission of rank 2. In this case, the precodinginformation for four antenna ports may be configured as shown in Table46.

TABLE 46 One codeword: Two codeword: Codeword 0 (/1) enabled, Codeword 0enabled, Codeword 1 (/0) disabled Codeword 1 enabled Bit field Bit fieldmapped mapped to index Message to index Message 0 1 layer in 1^(st)group 0 2 Layers in 1^(st) group 1 1 layer in 2^(nd) group 1 2 Layers in2^(nd) group 2 2 Layers in 1^(st) group 2 3 Layers 3 2 Layers in 2^(nd)group 3 4 Layers

The precoding information fields configured as shown in Tables 39 to 46may be included in a downlink control information (DCI) format. Theprecoding information field indicates that the control information to beincluded in the DCI format proposed by the present invention reuses the“precoding information field” of the existing DCI format 2A. That is,control information proposed by the present invention has a formatsimilar to the format of “precoding information” included in theexisting DCI format 2A and substantially indicates a group, to which alayer (or an antenna port) for transmitting transmission rank and databelongs, if only one codeword is enabled or if two codewords areenabled. Accordingly, the term control information may be changed. Forexample, the control information shown in Tables 39 to 46 may beincluded as control information indicating a group to which transmissionrank and layer belongs in the modified DCI format of the existing DCIformat 2A, which is proposed by the present invention, or a newlyproposed DCI format.

Embodiment 6

The present embodiment relates to a method of configuring controlinformation when MU-MIMO is applied within the same CDM group in thecase of indicating rank and layer used for transmission with respect totransmission of rank 1 and rank 2 and indicating transmission rankinformation with respect to transmission of rank 3 or more. Applicationof MU-MIMO within the same CDM group means that dedicated referencesignals for one or more layers of multiple users are transmitted on thesame CDM group (e.g., “C” of FIG. 8(a)).

For example, in MU-MIMO transmission applied within the same CDM group,it is assumed that each receiver may receive a maximum of 2 layers and atransmitter may transmit a maximum of four layers. In this case, thereis a need for an indication bit for distinguishing (or identifying) fourlayers for UEs which participate in MU-MIMO. In the case of applying theabove-described indication method for MIMO transmission, controlinformation may be provided such that a receiver recognizes statuses Ato D of Table 47. Although MU-MIMO transmission is performed within thegroup 1 in Table 47, this is for an arbitrary group and thus the presentinvention is not limited thereto.

TABLE 47 A-subgroup in Group 1 B-subgroup in Group 1 OCC #1 OCC #2 OCC#3 OCC #4 A 2-layers 2-layers B 1-layer 1-layer 2-layers C 2-layers1-layer 1-layer D 1-layer 1-layer 1-layer 1-layer E 1-layer 2-layers1-layer

Dedicated reference signals for four layers belonging to a predeterminedgroup may be distinguished (or identified) using four orthogonal codes.Four orthogonal codes may be divided into two subgroups (e.g.,A-subgroup and B-subgroup as shown in Table 47). For example, one groupmay be divided into two subgroups and the two subgroups arequasi-orthogonal to each other. Two layers may be distinguished (oridentified) using fully orthogonal sequences within one subgroup. Theorthogonal code for each layer may be determined according to an antennaport mapping rule of DMRS.

It is assumed that MU-MIMO is applied in transmission of rank 1 and 2,SU-MIMO is applied in transmission of rank 3 or more, and MU-MIMO isperformed only in one group (e.g., a group 1). In this case, the numberof REs used for DMRS may be restricted to 12. Thus, the precodinginformation field for eight antenna ports may be configured as shown inTable 48.

TABLE 48 One codeword: Two codeword: Codeword 0 (/1) enabled, Codeword 0enabled, Codeword 1 (/0) disabled Codeword 1 enabled Bit field Bit fieldmapped mapped to index Message to index Message 0 1 layer in subgroup-A0 2 Layers in subgroup-A 1 1 layer in subgroup-B 1 2 Layers insubgroup-B 2 2 Layers 2 3 Layers 3 3 Layers 3 4 Layers 4 4 Layers 4 5Layers 5 5 6 Layers 6 6 7 Layers 7 7 8 Layers

Meanwhile, when one codeword is enabled, MIMO transmission of rank 2, 3or 4 is possible. When one codeword is enabled, it is possible toindicate a subgroup, to which two layers are allocated, in order toenable MU-MIMO transmission in transmission of rank 2. In this case, inconsideration of the case in which MU-MIMO is applied within apredetermined group, the precoding information for eight antenna portsmay be configured as shown in Table 49.

TABLE 49 One codeword: Two codeword: Codeword 0 (/1) enabled, Codeword 0enabled, Codeword 1 (/0) disabled Codeword 1 enabled Bit field Bit fieldmapped mapped to index Message to index Message 0 1 layer in subgroup-A0 2 Layers in subgroup-A 1 1 layer in subgroup-B 1 2 Layers insubgroup-B 2 2 Layers in subgroup-A 2 3 Layers 3 2 Layers in subgroup-B3 4 Layers 4 3 Layers 4 5 Layers 5 4 Layers 5 6 Layers 6 6 7 Layers 7 78 Layers

The UE may perform encoding/decoding in consideration of the number ofREs for transmission of rank 1 or rank 2 (e.g., in the case in which atotal of 24 REs is used for DMRS transmission, only 12 REs are used whentransmission of rank 1 or 2 is performed).

Meanwhile, in the case in which MU-MIMO is applied in transmission ofranks 1 and 2 and SU-MIMO is applied in transmission of rank 3 or more,in consideration of the case in which MU-MIMO is applied within apredetermined group, the precoding information field for four antennaports may be configured as shown in Table 50.

TABLE 50 One codeword: Two codeword: Codeword 0 (/1) enabled, Codeword 0enabled, Codeword 1 (/0) disabled Codeword 1 enabled Bit field Bit fieldmapped mapped to index Message to index Message 0 1 layer in subgroup-A0 2 Layers in subgroup-A 1 1 layer in subgroup-B 1 2 Layers insubgroup-B 2 2 Layers 2 3 Layers 3 3 4 Layers

Meanwhile, when one codeword is enabled, it is possible to indicate agroup, to which two layers are allocated, in order to enable MU-MIMOtransmission in transmission of rank 2. In this case, in considerationof the case in which MU-MIMO is applied within a predetermined group,the precoding information for four antenna ports may be configured asshown in Table 51.

TABLE 51 One codeword: Two codeword: Codeword 0 (/1) enabled, Codeword 0enabled, Codeword 1 (/0) disabled Codeword 1 enabled Bit field Bit fieldmapped mapped to index Message to index Message 0 1 layer in subgroup-A0 2 Layers in subgroup-A 1 1 layer in subgroup-B 1 2 Layers insubgroup-B 2 2 Layers in subgroup-A 2 3 Layers 3 2 Layers in subgroup-B3 4 Layers

The precoding information fields configured as shown in Tables 48 to 51may be included in a downlink control information (DCI) format. Theprecoding information field indicates that the control information to beincluded in the DCI format proposed by the present invention reuses the“precoding information field” of the existing DCI format 2A. That is,control information proposed by the present invention has a formatsimilar to the format of “precoding information” included in theexisting DCI format 2A and, as described above, substantially indicatesa subgroup, to which a layer (or an antenna port) for transmittingtransmission rank and data belongs, if only one codeword is enabled orif two codewords are enabled. Accordingly, the term control informationmay be changed. For example, the control information shown in Tables 48to 51 may be included as control information indicating a subgroup towhich transmission rank and layer belongs in the modified DCI format ofthe existing DCI format 2A, which is proposed by the present invention,or a newly proposed DCI format.

In the case in which a group to which a MIMO transmission layer for a UEbelongs is indicated as in Embodiment 5, layers may be distinguished (oridentified) using two OCCs within the group. If one codeword istransmitted, antenna port, layer number or OCC information of an enabledcodeword may be acquired using an NDI bit of a disabled codeword. If twocodewords are enabled, two OCC resources available within the group maybe used.

In the case in which a subgroup to which a MIMO transmission layer for aUE belongs is indicated as in Embodiment 6, layers may be distinguished(or identified) using two OCCs within the subgroup. If one codeword istransmitted, antenna port, layer number or OCC information of an enabledcodeword may be acquired using an NDI bit of a disabled codeword. If twocodewords are enabled, two OCC resources available within the subgroupmay be used.

A method of transmitting and receiving a downlink signal in a wirelesscommunication system supporting MIMO transmission according to anembodiment of the present invention will be described with reference toFIG. 10. A downlink transmitter (e.g., a base station) of FIG. 10 maytransmit a downlink signal via eight transmit antennas and a downlinkreceiver (e.g., a UE) may receive a downlink signal from the downlinktransmitter. In addition, the downlink transmitter may be a relay fortransmitting an access downlink signal to a UE and the downlink receivermay be a relay for receiving a backhaul downlink signal from a basestation.

In step S1010, the downlink transmitter may transmit downlink controlinformation. Then, in step S1030, the downlink receiver may receive thedownlink control information. The downlink control information may beinformation for scheduling downlink data transmitted on N (1≤N≤8)layers. The downlink control information may be transmitted in a PDCCHDCI format.

In step S1020, the downlink transmitter may transmit downlink data andUE-specific reference signals (or DMRSs) transmitted on N layers basedon the downlink control information of step S1010. The downlink data andthe UE-specific reference signals may be multiplexed and transmitted ona PDCCH. Then, in step S1040, the downlink receiver may receive thedownlink data and the UE-specific reference signals.

In step S1050, the downlink receiver may demodulate the downlink datatransmitted on the N layers based on the UE-specific reference signals.

The downlink control information may include information for schedulingdownlink MIMO transmission in the case in which only one codeword isenabled or in the case in which two codewords are enabled. The downlinkcontrol information may include information indicating the number N oflayers to which enabled codewords are mapped. If the downlinktransmitter includes eight transmit antennas, 1≤N≤8. More specifically,transmission of 1 to 4 layers may be performed if only one codeword isenabled and transmission of 2 to 8 layers may be performed if twocodewords are enabled. The number of layers may indicate rank. Forexample, the downlink control information may include the controlinformation shown in Tables 40 to 44 or Tables 48 to 51.

Information indicating the number of layers in the downlink controlinformation may include information about codes for identifying theUE-specific reference signals, that is, information for distinguishingthe UE-specific reference signals transmitted at the same RE position.For example, as in the reference signal group “C” of FIG. 8(a), fourdifferent UE-specific reference signals transmitted at the same REposition may be included in one reference signal group, one referencegroup may be divided into two subgroups by information about the codesfor identifying the UE-specific reference signals, and one subgroup mayinclude two UE-specific reference signals divided by orthogonal codes.For example, as shown in Table 48, information indicating the number oflayers may have 3 bits and information indicating a subgroup to which alayer belongs (that is, information indicating codes for identifying theUE-specific reference signals) may be included together with respect tothe case in which one codeword is mapped to one layer and two codewordsare mapped to two layers. In addition, the downlink control informationmay further include information indicating a downlink MIMO transmissionantenna port.

The matters described in the various embodiments of the presentinvention above (that is, various DCI format configuration methodsproposed by the present invention) are equally applicable to the methodof transmitting and receiving the downlink signal of the presentinvention described with reference to FIG. 10. For example, as inEmbodiments 5 and 6, in a system supporting MIMO transmission of amaximum of rank 8, in the configuration of a DCI format for DMRS baseddata transmission with respect to each UE, a method of configuringcontrol information so as to simultaneously support SU-MIMO and MU-MIMO(that is, using one DCI format) is used. Such information may include atleast one of information indicating a group (that is, a reference signalposition) to which a dedicated reference signal allocated to a UEbelongs, information indicating transmission rank (the number of layers)of the UE, information indicating total transmission rank andinformation indicating a transmission layer of the UE. The informationindicating the transmission layer can distinguish (or identify) adedicated reference signal for demodulating data transmitted via thelayer (or the antenna port) and a dedicated reference signal fordemodulating data transmitted via another layer. Such controlinformation may be included in the modified format of the DCI format 2A,which is proposed by the present invention, and a new DCI format.

FIG. 11 is a diagram showing the configuration of a base station 1110and a UE 1120 according to an exemplary embodiment of the presentinvention.

Referring to FIG. 11, the base station 1110 according to the presentinvention may include a reception module 1111, a transmission module1112, a processor 1113, a memory 1114 and a plurality of antennas 1115.The use of the plurality of antennas 1115 means that the base stationsupports MIMO transmission. The reception module 1111 may receive avariety of signals, data and information from the UE in downlink. Thetransmission module 1112 may transmit a variety of signals, data andinformation to the UE in uplink. The processor 1113 may control theoverall operation of the base station 1110.

The base station 1110 according to the embodiment of the presentinvention may operate in a wireless communication system supportingdownlink MIMO transmission and support SU-MIMO or MU-MIMO transmission.The processor 1113 of the base station 1110 may be configured totransmit downlink control information including information indicatingthe number of layers, to which one or two enabled codewords of downlinkMIMO transmission are mapped, through the transmission module 1112. Theprocessor 1113 of the base station 1110 may be configured to transmitdownlink data transmitted on one or more layers and DMRSs of the one ormore layers through the transmission module 1111 based on downlinkcontrol information. The information indicating the number of layers mayfurther include information about codes for identifying DMRSs.

The processor 1113 of the base station 1110 processes informationreceived from the base station 1110 and information to be transmitted toan external device, and the memory 1114 stores the processed informationfor a predetermined time and may be replaced with a component such as abuffer (not shown).

Referring to FIG. 11, the UE 1120 according to the present invention mayinclude a reception module 1121, a transmission module 1122, a processor1123, a memory 1124 and a plurality of antennas 1125. The use of theplurality of antennas 1125 means that the UE supports MIMO transmission.The reception module 1121 may receive a variety of signals, data andinformation from the base station in downlink. The transmission module1122 may transmit a variety of signals, data and information to the basestation in uplink. The processor 1123 may control the overall operationof the UE 1120.

The UE 1120 according to the embodiment of the present invention mayoperate in a wireless communication system supporting downlink MIMOtransmission. The processor 1123 of the base station 1120 may beconfigured to receive downlink control information including informationindicating the number of layers, to which one or two enabled codewordsof downlink MIMO transmission are mapped, through the reception module1121. The processor 1123 of the base station 1120 may be configured toreceive downlink data transmitted on one or more layers and DMRSs of theone or more layers through the reception module 1121 based on downlinkcontrol information. The information indicating the number of layers mayfurther include information about codes for identifying DMRSs.

The processor 1123 of the UE 1120 processes information received fromthe UE 1120 and information to be transmitted to an external device, andthe memory 1124 stores the processed information for a predeterminedtime and may be replaced with a component such as a buffer (not shown).

In the wireless communication system supporting multiple carriers, thefollowing matters are commonly applicable to transmission of uplinkcontrol information from the UE 1120 to the base station 1110.Information about codes for identifying DMRSs may distinguish DMRSstransmitted at the same RE position and may be included in theinformation indicating the number of layers only when one codeword ismapped to one layer and when two codewords are mapped to two layers.Four different UE-specific reference signals transmitted at the same REposition may be included in one DMRS group, and one DMRS group isdivided into two subgroups by the information about the codes foridentifying the DMRSs, and one subgroup includes two DMRSs divided byorthogonal codes. In addition, the downlink control information mayfurther include information indicating antenna ports of the downlinkMIMO transmission. The information indicating the number of layers mayhave 3 bits.

The detailed configuration of the base station or the UE may beimplemented by equally applying the matters (that is, various DCI formatconfiguration methods proposed by the present invention) described inthe various embodiments of the present invention.

In the description of FIG. 11, the description of the base station 1110is equally applicable to an RN as a downlink transmitter or an uplinkreceiver and the description of the UE 1120 is equally applicable to anRN as a downlink receiver or an uplink transmitter.

The embodiments of the present invention can be implemented by a varietyof means, for example, hardware, firmware, software, or a combinationthereof.

In the case of implementing the present invention by hardware, thepresent invention can be implemented with application specificintegrated circuits (ASICs), Digital signal processors (DSPs), digitalsignal processing devices (DSPDs), programmable logic devices (PLDs),field programmable gate arrays (FPGAs), a processor, a controller, amicrocontroller, a microprocessor, etc.

If operations or functions of the present invention are implemented byfirmware or software, the present invention can be implemented in theform of a variety of formats, for example, modules, procedures,functions, etc. Software code may be stored in a memory unit so that itcan be driven by a processor. The memory unit is located inside oroutside of the processor, so that it can communicate with theaforementioned processor via a variety of well-known parts.

The detailed description of the exemplary embodiments of the presentinvention has been given to enable those skilled in the art to implementand practice the invention. Although the invention has been describedwith reference to the exemplary embodiments, those skilled in the artwill appreciate that various modifications and variations can be made inthe present invention without departing from the spirit or scope of theinvention described in the appended claims. For example, those skilledin the art may use each construction described in the above embodimentsin combination with each other. Accordingly, the invention should not belimited to the specific embodiments described herein, but should beaccorded the broadest scope consistent with the principles and novelfeatures disclosed herein.

The aforementioned embodiments are achieved by combination of structuralelements and features of the present invention in a predeterminedmanner. Each of the structural elements or features should be consideredselectively unless specified separately. Each of the structural elementsor features may be carried out without being combined with otherstructural elements or features. Also, some structural elements and/orfeatures may be combined with one another to constitute the embodimentsof the present invention. The order of operations described in theembodiments of the present invention may be changed. Some structuralelements or features of one embodiment may be included in anotherembodiment, or may be replaced with corresponding structural elements orfeatures of another embodiment. Moreover, it will be apparent that someclaims referring to specific claims may be combined with other claimsreferring to the other claims other than the specific claims toconstitute the embodiment or add new claims by means of amendment afterthe application is filed.

INDUSTRIAL APPLICABILITY

The above-described embodiments of the present invention are applicableto various mobile communication systems supporting MIMO transmission.

The invention claimed is:
 1. A method of receiving, at a user equipment(UE), a downlink signal from a base station in a wireless communicationsystem supporting downlink multiple-input multiple-output (MIMO)transmission, the method comprising: receiving downlink controlinformation including an indication field indicating one state among aplurality of states respectively indicating different number of layerswhere one or two enabled codewords of downlink MIMO transmission aremapped or identical number of layers where the one or two enabledcodewords of downlink MIMO transmission are mapped and differentinformation identifying UE-specific reference signals for demodulatingthe downlink data; and receiving downlink data based on the downlinkcontrol information.
 2. The method of claim 1, wherein the one stateindicated by the indication field indicates that the number of layers is1, 2, 3 or 4 when one codeword is enabled, and wherein the one stateindicated by the indication field indicates the number of layers is 2,3, 4, 5, 6, 7 or 8 when two codewords are enabled.
 3. The method ofclaim 1, wherein when one codeword is enabled and the one stateindicated by the indication field indicates that the number of layers is1, the one state further indicates different information identifyingUE-specific reference signals for demodulating the downlink data fromanother state among the plurality of states.
 4. The method of claim 1,wherein when two codewords are enabled and the one state indicated bythe indication field indicates that the number of layers is 2, the onestate further indicates different information identifying UE-specificreference signals for demodulating the downlink data from another stateamong the plurality of states.
 5. The method of claim 1, wherein thenumber of a plurality of states is up to
 8. 6. A user equipment (UE) forreceiving downlink signals from a base station in a wirelesscommunication system supporting multiple-input multiple-output (MIMO)transmission, the UE comprising: a receiver that receives downlinksignals from the base station; and a processor that controls the UEincluding the receiver, wherein the processor: receives, through thereceiver, downlink control information including an indication fieldindicating one state among a plurality of states respectively indicatingdifferent number of layers where one or two enabled codewords ofdownlink MIMO transmission are mapped or identical number of layerswhere the one or two enabled codewords of downlink MIMO transmission aremapped and different information identifying UE-specific referencesignals for demodulating the downlink data, and, receives, through thereceiver, downlink data based on the downlink control information. 7.The UE of claim 6, wherein the one state indicated by the indicationfield indicates that the number of layers is 1, 2, 3 or 4 when onecodeword is enabled, and wherein the one state indicated by theindication field indicates the number of layers is 2, 3, 4, 5, 6, 7 or 8when two codewords are enabled.
 8. The UE of claim 6, wherein when onecodeword is enabled and the one state indicated by the indication fieldindicates that the number of layers is 1, the one state furtherindicates different information identifying UE-specific referencesignals for demodulating the downlink data from another state among theplurality of states.
 9. The UE of claim 6, wherein when two codewordsare enabled and the one state indicated by the indication fieldindicates that the number of layers is 2, the one state furtherindicates different information identifying UE-specific referencesignals for demodulating the downlink data from another state among theplurality of states.
 10. The UE of claim 6, wherein the number of aplurality of states is up to
 8. 11. A method of transmitting, at a basestation, a downlink signal to a user equipment (UE) in a wirelesscommunication system supporting downlink multiple-input multiple-output(MIMO) transmission, the method comprising: transmitting downlinkcontrol information including an indication field indicating one stateamong a plurality of states respectively indicating different number oflayers where one or two enabled codewords of downlink MIMO transmissionare mapped or identical number of layers where the one or two enabledcodewords of downlink MIMO transmission are mapped and differentinformation identifying UE-specific reference signals for demodulatingthe downlink data; and transmitting downlink data based on the downlinkcontrol information.
 12. The method of claim 11, wherein the one stateindicated by the indication field indicates that the number of layers is1, 2, 3 or 4 when one codeword is enabled, and wherein the one stateindicated by the indication field indicates the number of layers is 2,3, 4, 5, 6, 7 or 8 when two codewords are enabled.
 13. The method ofclaim 11, wherein when one codeword is enabled and the one stateindicated by the indication field indicates that the number of layers is1, the one state further indicates different information identifyingUE-specific reference signals for demodulating the downlink data fromanother state among the plurality of states.
 14. The method of claim 11,wherein when two codewords are enabled and the one state indicated bythe indication field indicates that the number of layers is 2, the onestate further indicates different information identifying UE-specificreference signals for demodulating the downlink data from another stateamong the plurality of states.
 15. The method of claim 11, wherein thenumber of a plurality of states is up to
 8. 16. A base station fortransmitting downlink signals to a user equipment (UE) in a wirelesscommunication system supporting multiple-input multiple-output (MIMO)transmission, the base station comprising: a transmitter that transmitsdownlink signals to the UE; and a processor that controls the basestation including the transmitter, wherein the processor: transmits,through the transmitter, downlink control information including anindication field indicating one state among a plurality of statesrespectively indicating different number of layers where one or twoenabled codewords of downlink MIMO transmission are mapped or identicalnumber of layers where the one or two enabled codewords of downlink MIMOtransmission are mapped and different information identifyingUE-specific reference signals for demodulating the downlink data, andtransmits, through the transmitter, downlink data based on the downlinkcontrol information.
 17. The base station of claim 16, wherein the onestate indicated by the indication field indicates that the number oflayers is 1, 2, 3 or 4 when one codeword is enabled, and wherein the onestate indicated by the indication field indicates the number of layersis 2, 3, 4, 5, 6, 7 or 8 when two codewords are enabled.
 18. The basestation of claim 16, wherein when one codeword is enabled and the onestate indicated by the indication field indicates that the number oflayers is 1, the one state further indicates different informationidentifying UE-specific reference signals for demodulating the downlinkdata from another state among the plurality of states.
 19. The basestation of claim 16, wherein when two codewords are enabled and the onestate indicated by the indication field indicates that the number oflayers is 2, the one state further indicates different informationidentifying UE-specific reference signals for demodulating the downlinkdata from another state among the plurality of states.
 20. The basestation of claim 16, wherein the number of a plurality of states is upto 8.